Provide more organized output and sorting

Closes https://github.com/gjtorikian/html-proofer/issues/72.

Would appreciate some :eyes: from @penibelst when you have the :clock1030:.

I could not figure out how to do neat ascii separators, so I went with `  *` instead. I also apologize for the massive changes in how tests are written, it was a refactor that needed to happen.