Add ability to comment on pull request

[Aaron1011]

It would be great if hub had the ability to create a comment on the pull request for the current branch (e.g. hub comment "A Comment")

[mislav]

Yeah, there is never enough of some sort of code reviewing features. However, since I'm keeping hub strictly as a git tool, and not for anything else, I think such features would best be developed by someone else as a separate project.

It's trivial to submit a PR comment via the API but what's not trivial is to create a good conversation & reviewing experience in the command-line. Right now I would still go to the website to leave a comment, mostly because I would like to read other comments as well, comment on diffs, etc.