Add --quiet option for error-only output

[paulcpederson]

I'm running a11y on an entire folder of files from the command line. It would really help me if there was a way to only output errors. I'm thinking that if all the rules pass for a page, it would just log the page with a checkmark. Example output:

$ ally **/*.html

✔ file1.html
✔ file2.html
✖ file3.html

  ✖ The purpose of each link should be clear from the link text

   body > .panel.panel-white.article.article-red > A

✔ file4.html

Let me know if you'd be interested in a feature like this, happy to pr.