Feature Request: JUnit.xml format

[wscourge]

It is widely accepted by CI services to integrate with their UI. I think it'd be nice. I'd like it to be used this way:

bundle exec rails_best_practices --format junit --output-file rbp.xml

Thanks, keep up the good work.

[wscourge]

If you accept PRs, I can give it a shot.

[flyerhzm]

@wscourge I appreciate it if you could create a PR.