Add method to comment on revisions

[jamesmontalvo3]

Input from Derrick: Often people make revisions that I want to leave small comments on, and end up sending emails/IMs to transmit the info. It'd be nice if it was baked into the review process. Furthermore, if someone made a change I disagreed with it'd be nice to be able to revert it and leave a comment.

Of course some of this comm could be done on the discussion page (this is how wikipedia does it), but for small bits of text it may be possible and may be a cleaner workflow to bake this comm into the pending reviews process.