Rename customize group for faces to rebase-mode-faces

This is to prepare the way for a (non-face) customize group for
rebase-mode itself.
Add option to auto-advance to the next line after selecting action

It's convenient in some situations to auto-advance point to the next
line after selecting an action for a given commit in the rebase-todo
buffer (after all, presumably you're done selecting the action for
that line). Add an option to automatically advance after selecting an
action.