Add Command hook - #55

Thanks for the PR!

This functionality is built into pre-commit after version 0.5.0 (however I have yet to document it). You can read more about it from the pull request: pre-commit/pre-commit#226

The tl;dr that I hope to commit to the docs soon is you can do something like this:

-   repo: local
    hooks:
    -   id: system-local-hook
        name: Just runs ls because why not
        entry: ls
        language: system
        files: ''

Great to see, you guys are working on it. This feature is must have, so thought of building one. But anyways, please feel free to close it, If nothing new to integrate in your system hook.

local hooks are now documented here: http://pre-commit.com/#repository-local-hooks