avy-goto-char-timer: indicate somehow when match is unique

[asmeurer]

With avy-goto-char-timer, it would be useful if it somehow indicated visually when the match is unique. Perhaps it could flash the selection (is this possible?). Or it could ring the bell, although maybe that wouldn't be ideal because the bell usually means there is an error.

This is somewhat related to #284, however, I don't think it's a good idea to just jump as soon as the match is uniquely determined, because then avy would end unexpectedly and you would continue typing, which would end up putting the text in the buffer next to your match. However, it would be nice to know that I can stop typing and let the timer run out once I no longer need to.

[talw]

Perhaps it could flash the selection (is this possible?). Or it could ring the bell, although maybe that wouldn't be ideal because the bell usually means there is an error.

Perhaps it could use a different (second) face.