preface comments with cop names if config has `ShowCopNames: true`

This is following https://github.com/thoughtbot/hound/issues/264.

I'm not sure if the `options` attribute of `Rubocop::Cop::Team` is meant to be set in this way, but it does get the job done with minimal code. If you think it's a good implementation / design, I will update the README.

I also considered modifying https://github.com/thoughtbot/hound/blob/248cf2f9339afa9addbdf207e979f160ae68bff1/app/models/style_checker.rb#L23-L27 where the actual messages are passed into the `LineViolation` model but we still have access (I think) to the cop name. That would give us more flexibility as to how to display the cop name.
We are actually against guard clause. That rule has been recently disabled in master, but hasn't been deployed to production yet.