Differentiate between rubocop severities

[PChambino]

Everything is being highlighted as errors. It would be nice to differentiate between the severities that rubocop has.

I separated them like this:

• Severities [:refactor, :convention, :warning] are highlighted as warnings.
• Severities [:error, :fatal] are highlighted as errors.

[aparajita]

Very good point, I could have sworn I had done that from the beginning. Merged, thank you!