Remove flags

The ability to remove flags on an item was a feature that I needed. 

I'm doing this by setting unflag = '1' on flaggable, with a before save callback that calls destroy all flaggings if unflag is set, example: https://gist.github.com/gordonbisnor/9ceb1bdaff6748891e2f

Wondered if you would want to merge this into the original project. 

Thanks for the great gem!