Improving avy-goto-char-timer

Until recently, the avy command avy-goto-char-timer read one char or two chars if the latter was typed within avy-timeout-seconds.  This command is the one I use most frequently of all avy commands (you can say, it’s basically the only one I use except for avy-goto-line), so I’ve improved it with this commit and this pull request which I guess will be accepted anytime soon.

What these two commits implement:

1. Now avy-goto-char-timer may read one or as many chars as you want given that the respective next char is typed within avy-timeout-seconds.
2. DEL deletes the last character from the input, so that you can fix typos easily without having to abort and invoke avy-goto-char-timer once again.
3. RET immediately stops reading more chars without waiting for another avy-timeout-seconds.
4. Most importantly: the matches of the current input chars are immediately highlighted to give you direct visual feedback.

Here’s a screencast of the new behavior.

BTW, thanks a lot for camcorder.el, Artur!

UPDATE: The PR has already been merged, so you can now use this feature by updating your avy copy from MELPA.