Add more Git hooks, delegate to .local convention

The `pre-commit` and `prepare-commit-msg` git hooks can be used for some
interesting things such as linting files. For example:

https://github.com/croaky/dotfiles/commit/b3bc7d3f0dff46a47697c8c81a8352b76eb0e3a5
@jferris I'm the one that 'established' that convention, but the more I deal with it, the more I wish I'd have just appended `.local` to the end of the hook file, not the template directory.

Do we think its too late to establish the `~/.git_template/hooks/{pre,post}-*.local` convention?

I have a blog post in the works (https://github.com/thoughtbot/robots.thoughtbot.com/pull/359) that will hopefully cement whichever convention we establish.