Add integration for pre-commit.com (#104)

Fixes #103

.pre-commit-config.yaml

@@ -0,0 +1,19 @@
+# Note: don't use this config for your own repositories. Instead, see
+# "Version control integration" in README.md.
+-   repo: local
+    hooks:
+    - id: black
+      name: black
+      language: system
+      entry: python3 -m black
+      files: ^(black|setup|tests/test_black)\.py$
+    - id: flake8
+      name: flake8
+      language: system
+      entry: flake8
+      files: ^(black|setup|tests/test_black)\.py$
+    - id: mypy
+      name: mypy
+      language: system
+      entry: mypy
+      files: ^(black|setup|tests/test_black)\.py$

.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+-   id: black
+    name: black
+    entry: black
+    language: python
+    language_version: python3.6
+    types: [python]

CONTRIBUTING.md

@@ -30,6 +30,7 @@ myself and CentOS at work.
 Install all development dependencies using:
 ```
 $ pipenv install --dev
+$ pre-commit install
 ```
 If you haven't used `pipenv` before but are comfortable with virtualenvs,
 just run `pip install pipenv` in the virtualenv you're already using and
@@ -41,10 +42,6 @@ Before submitting pull requests, run tests with:
 $ python setup.py test
 ```
 
-Also run mypy and flake8 on `black.py` and `test_black.py`.  Travis will
-run all that for you but if you left any errors here, it will be quicker
-and less embarrassing to fix them locally ;-)
-
 
 ## Hygiene
 

Pipfile

@@ -9,6 +9,7 @@ click = "*"
 setuptools = ">=38.6.0"
 
 [dev-packages]
+pre-commit = "*"
 coverage = "*"
 flake8 = "*"
 flake8-bugbear = "*"

README.md

@@ -357,6 +357,28 @@ affect your use case.
 This can be used for example with PyCharm's [File Watchers](https://www.jetbrains.com/help/pycharm/file-watchers.html).
 
 
+## Version control integration
+
+Use [pre-commit](https://pre-commit.com/). Once you [have it
+installed](https://pre-commit.com/#install), add this to the
+`.pre-commit-config.yaml` in your repository:
+```yaml
+repos:
+-   repo: https://github.com/ambv/black
+    rev: stable
+    hooks:
+    - id: black
+      args: [--line-length=88, --safe]
+      python-version: python3.6
+```
+Then run `pre-commit install` and you're ready to go.
+
+`args` in the above config is optional but shows you how you can change
+the line length if you really need to.  If you're already using Python
+3.7, switch the `python-version` accordingly. Finally, `stable` is a tag
+that is pinned to the latest release on PyPI.  If you'd rather run on
+master, this is also an option.
+
 ## Testimonials
 
 **Dusty Phillips**, [writer](https://smile.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=dusty+phillips):