I love that there is now a way to add and remove checks from the cli! (:

I think a nice improvement would be some reasonable defaults. Instead of always having to specify `<git|yaml> <checks|warnings>` it would be nice if defaults were used:

    pre-commit enable jshint

And override-able:

    pre-commit enable --warn whitespace

These changes are not necessary for this PR, but I can create an issue to track the discussion or open up a PR with a proposed solution.