Before-commit

===================
- Add support for the `commit-msg` git hook

===================
- Recover from invalid python virtualenvs

===================
- Work around a windows-specific virtualenv bug pypa/virtualenv1062
This failure mode was introduced in 0.15.1

===================
- Use a more intelligent default language version for python

===================
- Add `types` and `exclude_types` for filtering files. These options take
an array of "tags" identified for each file. The tags are sourced from
[identify](https://github.com/chriskuehl/identify). One can list the tags
for a file by running `identify-cli filename`.
- `files` is now optional (defaulting to `''`)
- `always_run` + missing `files` also defaults to `files: ''` (previously it
defaulted to `'^$'` (this reverses e150921c).

===================
- Expose `--origin` and `--source` as `PRE_COMMIT_ORIGIN` and
`PRE_COMMIT_SOURCE` environment variables when running as `pre-push`.