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.
Let me know if you think the style violations here should be addressed.

Mainly interested, though, in whether you like the feature and/or its implementation -- I think providing the option to display cop names is useful, and makes it easier for developers to decide on and encode their conventions. But if we're going to control that using a rubocop YAML file, maybe I should be making a pull request to Rubocop to allow config file control of message formatting :)