Support comment hiding

[jsok]

GitHub allows you to hide a comment (with a reason), e.g.:

This is useful since you may not want to readily see old comments, but also don't want to delete them (since they may be useful to reference later).

[aaronjwhiteside]

Great suggestion!

[hetile-ssense]

This feature would be very useful. We have jenkins posting live update on commits and it would be great if we could simply get a list of previous comments and then mark the previous build as outdated automatically as suggested.