Implement `uninstall` action

[wevtimoteo]

As we register pre-commit hook in .git/hooks, there is a possibility that user doesn't want to use git-pair anymore, so we should add ability to remove created hook by running:

git pair uninstall

It will actually:

• Remove all pair.coauthor entries from .git/config (we can use our git pair stop implementation)
• rm .git/hooks/pre-commit[-append-coauthors]

I was wondering if we should call it unregister-hooks instead of uninstall. WDYT?

This isn't mapped in README, so remember to update it.