feat(yamllint): include for this repo and apply rules throughout

* Semi-automated using `ssf-formula` (v0.5.0)
* Fix errors shown below:

```bash
nginx-formula$ $(grep "\- yamllint" .travis.yml | sed -e "s:^\s\+-\s\(.*\):\1:")
pillar.example
  5:1       warning  missing document start "---"  (document-start)
  6:89      error    line too long (110 > 88 characters)  (line-length)
  7:89      error    line too long (99 > 88 characters)  (line-length)
  19:89     error    line too long (132 > 88 characters)  (line-length)
  35:25     warning  truthy value should be one of [false, true]  (truthy)
  36:89     error    line too long (106 > 88 characters)  (line-length)
  40:89     error    line too long (115 > 88 characters)  (line-length)
  43:16     warning  truthy value should be one of [false, true]  (truthy)
  44:89     error    line too long (105 > 88 characters)  (line-length)
  48:16     warning  truthy value should be one of [false, true]  (truthy)
  54:14     warning  too few spaces before comment  (comments)
  57:13     warning  truthy value should be one of [false, true]  (truthy)
  57:18     warning  too few spaces before comment  (comments)
  58:14     warning  too few spaces before comment  (comments)
  60:5      warning  missing starting space in comment  (comments)
  60:89     error    line too long (96 > 88 characters)  (line-length)
  61:89     error    line too long (90 > 88 characters)  (line-length)
  62:89     error    line too long (90 > 88 characters)  (line-length)
  63:89     error    line too long (94 > 88 characters)  (line-length)
  64:5      warning  missing starting space in comment  (comments)
  64:89     error    line too long (96 > 88 characters)  (line-length)
  68:9      error    wrong indentation: expected 10 but found 8  (indentation)
  76:9      error    wrong indentation: expected 10 but found 8  (indentation)
  86:9      error    wrong indentation: expected 10 but found 8  (indentation)
  90:14     warning  too few spaces before comment  (comments)
  90:89     error    line too long (104 > 88 characters)  (line-length)
  92:6      warning  missing starting space in comment  (comments)
  92:89     error    line too long (106 > 88 characters)  (line-length)
  94:89     error    line too long (104 > 88 characters)  (line-length)
  97:6      warning  missing starting space in comment  (comments)
  97:89     error    line too long (106 > 88 characters)  (line-length)
  100:62    warning  too few spaces before comment  (comments)
  100:89    error    line too long (121 > 88 characters)  (line-length)
  101:62    warning  comment not indented like content  (comments-indentation)
  101:89    error    line too long (116 > 88 characters)  (line-length)
  102:89    error    line too long (112 > 88 characters)  (line-length)
  103:89    error    line too long (99 > 88 characters)  (line-length)
  105:89    error    line too long (124 > 88 characters)  (line-length)
  106:8     warning  missing starting space in comment  (comments)
  106:89    error    line too long (117 > 88 characters)  (line-length)
  112:89    error    line too long (97 > 88 characters)  (line-length)
  120:31    warning  missing starting space in comment  (comments)
  123:89    error    line too long (95 > 88 characters)  (line-length)
  127:13    error    wrong indentation: expected 14 but found 12  (indentation)
  151:33    warning  too few spaces before comment  (comments)
  151:89    error    line too long (94 > 88 characters)  (line-length)
  152:22    warning  too few spaces before comment  (comments)
  152:89    error    line too long (90 > 88 characters)  (line-length)
  153:21    warning  too few spaces before comment  (comments)
  153:89    error    line too long (101 > 88 characters)  (line-length)
  154:22    warning  too few spaces before comment  (comments)
  155:18    warning  too few spaces before comment  (comments)
  155:89    error    line too long (103 > 88 characters)  (line-length)
  163:89    error    line too long (100 > 88 characters)  (line-length)
  164:89    error    line too long (106 > 88 characters)  (line-length)
  165:18    warning  truthy value should be one of [false, true]  (truthy)
  167:89    error    line too long (109 > 88 characters)  (line-length)
  170:10    warning  missing starting space in comment  (comments)
  172:10    warning  missing starting space in comment  (comments)
  172:89    error    line too long (120 > 88 characters)  (line-length)
  173:10    warning  missing starting space in comment  (comments)
  173:89    error    line too long (118 > 88 characters)  (line-length)
  174:89    error    line too long (120 > 88 characters)  (line-length)
  175:20    warning  truthy value should be one of [false, true]  (truthy)
  175:89    error    line too long (102 > 88 characters)  (line-length)
  177:89    error    line too long (100 > 88 characters)  (line-length)
  178:89    error    line too long (101 > 88 characters)  (line-length)
  182:13    error    wrong indentation: expected 14 but found 12  (indentation)
  184:15    error    wrong indentation: expected 16 but found 14  (indentation)
  186:15    error    wrong indentation: expected 16 but found 14  (indentation)
  189:15    error    wrong indentation: expected 16 but found 14  (indentation)
  193:9     warning  comment not indented like content  (comments-indentation)
  196:13    error    wrong indentation: expected 14 but found 12  (indentation)
  198:15    error    wrong indentation: expected 16 but found 14  (indentation)
  202:15    error    wrong indentation: expected 16 but found 14  (indentation)
  206:9     warning  comment not indented like content  (comments-indentation)
  218:16    warning  too few spaces before comment  (comments)
  218:89    error    line too long (96 > 88 characters)  (line-length)
  219:18    warning  truthy value should be one of [false, true]  (truthy)
  223:57    warning  too few spaces before comment  (comments)
  223:89    error    line too long (111 > 88 characters)  (line-length)
  224:57    warning  comment not indented like content  (comments-indentation)
  224:89    error    line too long (118 > 88 characters)  (line-length)
  226:89    error    line too long (90 > 88 characters)  (line-length)
  236:18    warning  truthy value should be one of [false, true]  (truthy)
  245:89    error    line too long (108 > 88 characters)  (line-length)
  252:89    error    line too long (104 > 88 characters)  (line-length)
  305:89    error    line too long (115 > 88 characters)  (line-length)
  306:32    error    trailing spaces  (trailing-spaces)

test/salt/default/pillar/nginx.sls
  1:1       error    too many blank lines (1 > 0)  (empty-lines)
  7:1       warning  missing document start "---"  (document-start)
  11:9      error    wrong indentation: expected 10 but found 8  (indentation)
  23:18     warning  truthy value should be one of [false, true]  (truthy)
  24:18     warning  truthy value should be one of [false, true]  (truthy)
  28:18     warning  truthy value should be one of [false, true]  (truthy)
  31:13     error    wrong indentation: expected 14 but found 12  (indentation)
  33:15     error    wrong indentation: expected 16 but found 14  (indentation)
  36:15     error    wrong indentation: expected 16 but found 14  (indentation)
  38:1      error    too many blank lines (1 > 0)  (empty-lines)
```

.travis.yml

@@ -3,7 +3,7 @@
 ---
 stages:
   - test
-  - commitlint
+  - lint
   - name: release
     if: branch = master AND type != pull_request
 
@@ -45,16 +45,21 @@ script:
 
 jobs:
   include:
-    # Define the commitlint stage
-    - stage: commitlint
+    # Define the `lint` stage (runs `yamllint` and `commitlint`)
+    - stage: lint
       language: node_js
       node_js: lts/*
       before_install: skip
       script:
+        # Install and run `yamllint`
+        - pip install --user yamllint
+        # yamllint disable-line rule:line-length
+        - yamllint -s . .yamllint pillar.example test/salt/default/pillar/nginx.sls
+        # Install and run `commitlint`
         - npm install @commitlint/config-conventional -D
         - npm install @commitlint/travis-cli -D
         - commitlint-travis
-    # Define the release stage that runs semantic-release
+    # Define the release stage that runs `semantic-release`
     - stage: release
       language: node_js
       node_js: lts/*

.yamllint

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
+---
+# Extend the `default` configuration provided by `yamllint`
+extends: default
+
+# Files to ignore completely
+# 1. All YAML files under directory `node_modules/`, introduced during the Travis run
+ignore: |
+  node_modules/
+
+rules:
+  line-length:
+    # Increase from default of `80`
+    # Based on https://github.com/PyCQA/flake8-bugbear#opinionated-warnings (`B950`)
+    max: 88