Therapist

- Added a new plugin system which allows for custom linting plugin to be
written specifically for Therapist.
- Uses [colorama](https://github.com/tartley/colorama) to handle ANSI
colors.
- Added the `--no-color` flag to disable colors from being printed in
the output.
- Added the `--plugin` option to the `run` command to allow configured
plugins to be run individually.

- Added the `--junit-xml` flag to allow for a JUnit XML style report to
be output to a given path.
- Added the `--quiet` flag that suppresses output from the `run`
command.

- Fix some decoding issues that occur in Python 2.7.
- Using the `--include-unstaged` flag now has the side-effect of setting
`--include-unstaged-changes` as well.
- If for some reason an error occurs while executing an action we now
ensure that any stashed changes are restored.

- Pre-commit hook is now a bash script that calls the run command from
the CLI.
- CLI can now identify the Therapist pre-commit hook during install and
uninstall.
- Support for retaining existing pre-commit hook when installing
Therapist pre-commit hook.
- Running actions no longer errors out when there files have been
modified after they've been staged. Actions are now run against the
state of the file that has been cached in the index unless the
`--include-unstaged-changes` flag is used in which case actions are
run against the files in their current state.
- Added the `--use-tracked-files` flag, which allows you to run actions
against all the files that are being tracked by the repo.