Auto advance to next line after selecting an action

This pull request contains two commits:

1. Minor refactor (put a -faces suffix on the existing rebase-mode customize group)
1. Auto-advance to the next line after choosing an action for a given line (with associated option to toggle). This cuts out one keystroke (`n`, to go to the next line in the rebase-todo list) for the common case of moving to the next line after selecting an action.