Implement Markdown trailing space line break preservation

Markdown uses two or more trailing spaces on a line to indicate a forced
line break `<br/>` - these will be preserved for files with a markdown
extension (default = `.md` or `.markdown`).

Add `--markdown-linebreak-ext=X,Y` to add extensions (`*` matches any),
and `--no-markdown-linebreak-ext` to disable this feature.

If you want to set specific extension `foo` only (and not md/markdown),
use `--no-markdown-linebreak-ext --markdown-linebreak-ext=foo`

Tries to prevent --markdown-linebreak-ext from eating filenames as if they were
extensions by rejecting any with '.' or '/' (or even Windows-style '\' or ':')

Update README.md to include information on these arguments as well as
arguments added to other hooks

Add extensive tests using pytest.mark.parametrize

test that `txt` file is not considered as 'txt' extension
test that `.txt` file is not considered as 'txt' extension

The latter is the (correct) behavior of os.path.splitext(), and an example
of why it is better to use the libraries than to mangle strings yourself.

README.md

@@ -23,23 +23,34 @@ Add this to your `.pre-commit-config.yaml`
 
 ### Hooks available
 
-- `autopep8-wrapper` - Runs autopep8 over python source. (You'll want `args: ['-i]` when using this hook, see `.pre-commit-config.yaml` for an example.)
+- `autopep8-wrapper` - Runs autopep8 over python source.
+    - Ignore PEP 8 violation types with `args: ['-i', '--ignore=E000,...']`,
+      see `.pre-commit-config.yaml` in this repository for an example.
 - `check-added-large-files` - Prevent giant files from being committed.
-- `check-case-conflict` - Check for files that would conflict in case-insensitive filesystems.
-- `check-docstring-first` - Checks a common error of defining a docstring after code.
+    - Specify what is "too large" with `args: ['--maxkb=123']` (default=500kB).
+- `check-case-conflict` - Check for files with names that would conflict on a
+  case-insensitive filesystem like MacOS HFS+ or Windows FAT.
+- `check-docstring-first` - Checks for a common error of placing code before
+  the docstring.
 - `check-json` - Attempts to load all json files to verify syntax.
 - `check-merge-conflict` - Check for files that contain merge conflict strings.
 - `check-xml` - Attempts to load all xml files to verify syntax.
 - `check-yaml` - Attempts to load all yaml files to verify syntax.
 - `debug-statements` - Check for pdb / ipdb / pudb statements in code.
-- `detect-private-key` - Checks for the existence of private keys
-- `double-quote-string-fixer` - This hook replaces double quoted strings with single quoted strings
+- `detect-private-key` - Checks for the existence of private keys.
+- `double-quote-string-fixer` - This hook replaces double quoted strings
+  with single quoted strings.
 - `end-of-file-fixer` - Makes sure files end in a newline and only a newline.
-- `flake8` - Run flake8 on your python files
-- `name-tests-test` - Assert that files in tests/ end in _test.py
-- `pyflakes` - Run pyflakes on your python files
+- `flake8` - Run flake8 on your python files.
+- `name-tests-test` - Assert that files in tests/ end in `_test.py`.
+    - Use `args: ['--django']` to match `test*.py` instead.
+- `pyflakes` - Run pyflakes on your python files.
 - `requirements-txt-fixer` - Sorts entries in requirements.txt
 - `trailing-whitespace` - Trims trailing whitespace.
+    - Markdown linebreak trailing spaces preserved for `.md` and`.markdown`;
+      use `args: ['--markdown-linebreak-ext=txt,text']` to add other extensions,
+      `args: ['--markdown-linebreak-ext=*']` to preserve them for all files,
+      or `args: ['--no-markdown-linebreak-ext']` to disable and always trim.
 
 ### As a standalone package