Add devil-exit-key for easy exit from repeating commands

[fbrosda]

Hi, I hope you don't mind, that I create another pull request. The change contains a new variable devil-exit-key, which can be set to some key to exit a repeatable key sequence. This is useful, if you start e.g. with , n and then want to one of the characters, which would otherwise map to one in the list of devil-repeatable keys.

The idea is taken from repeat-exit-key, which serves the same purpose for the repeat mode.

s.a. https://www.gnu.org/software/emacs/manual/html_node/emacs/Repeating.html