Add cmd-enter keyboard shortuct for submitting comments.

I'm not fully pleased with how this is done, but it does work.
There should probably be a better shortcut menu display that
will show the correct thing for the OS (e.g. cmd for OSX), and
it'd be cleaner if the shortcut stuff were refactored to allow
for meta key combinations in the normal callbacks.

The idea is from https://github.com/dewski/cmd-enter