show coverage information in pull-request diffs #2567

It would be great if the pull requests could be annotated to color the pull request diff based on the coverage (green = covered, red = not covered).

In Javascript one could use Istanbul, in Java, that could be based on coveralls data.

See also:

• Example: https://user-images.githubusercontent.com/2585460/33632250-1e383134-d9db-11e7-8ac8-e483a836064a.png
• Github unofficial issue: https://github.com/isaacs/github/issues/960