Added ability to update by changenum

Some SMCs (like Perforce) may be able to update reviews without having to compare summaries and descriptions by using the changenum directly. This works great for responding to review comments on unsubmitted changelists.

I also added an "update-or-post" option to allow a single Perforce custom tool (command line) to take care of both,

I doubt this is laid out exactly as you'd prefer, but I wanted to get comments and let people know I'd figured out a way to make this work.