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.
Use a guard clause instead of wrapping the code inside a conditional expression.