add @wordpress/scripts package (#363)

* add @wordpress/scripts package

* Only lint style.css and the files in the assets folder

* Add `.stylelintrc.json` file excluding the `no-descending-specificity` rule.

* Remove erronous trailing comma

* Add `font-family-no-missing-generic-family-keyword` rule

* use the ES5 ruleset

* Use the right ES5 rulesets

* Use the browser environment

.eslintrc

@@ -0,0 +1,6 @@
+{
+	"extends": [ "plugin:@wordpress/eslint-plugin/es5" ],
+	"env": {
+		"browser": true
+	}
+}

.stylelintrc.json

@@ -0,0 +1,7 @@
+{
+	"extends": "stylelint-config-wordpress",
+	"rules": {
+		"font-family-no-missing-generic-family-keyword": null,
+		"no-descending-specificity": null
+	}
+}

.travis.yml

@@ -29,6 +29,7 @@ allow_failures:
 
 before_install:
     - composer install
+    - npm install
 
 script:
   # Validate the composer.json file.
@@ -40,3 +41,9 @@ script:
 
   # Check the code against the standards as documented in the phpcs.xml.dist file.
   - if [[ "$PHPCS" == 1 ]]; then composer checkcs; fi
+
+  # Run the theme through lint:js checker as defined in @wordpress/scripts package.
+  - if [[ "$PHPCS" == 1 ]]; then npm run lint:js; fi
+
+  # Run the theme through lint:css checker as defined in @wordpress/scripts package.
+  - if [[ "$PHPCS" == 1 ]]; then npm run lint:css; fi