Group errors by rule violation

[pmcelhaney]

I often come across dozens or more violations, most of which are the same rule violated over and over. I would like to start with a summary of which rules were violated, and how often, and then drill down into the individual violations.

I plan on implementing the feature and opening a pull request this week.

[pmcelhaney]

I said I was going to work on it this week but didn't get a chance to start. Also, having gotten some experience with the code, I'm not keen to keep adding to the auditor UI, which consists of a lot of direct DOM manipulation. I would rather start by rewriting the UI using a lightweight framework like React or Vue.