breaking changes to provide preference groups

This is an update of the previous pull request, which I closed due to messiness.

Please see the updated README at [jnelson/QuickThemes](https://github.com/jnelson/QuickThemes#readme) for an overview of how the functionality and preference format would change.

Let me know whether you'll consider pulling or whether I should just create a new package (maybe called "QuickPrefs", with credit to chrislongo/QuickThemes prominently noted).

Thanks!