Support unread-command-events

When emacs commands change unread-command-events, you do not wish to run the mc post-command-hook, it can add multiple instances of the unread command events to the queue.  Some notable packages like isearch use unread-command-events

I found the bug when debugging mc in combination with multiple-cursors mode.  I don't need the fix, but think some other functions may become more compatible if you apply the fix.

See

https://github.com/ergoemacs/ergoemacs-mode/issues/198
https://github.com/ergoemacs/ergoemacs-mode/issues/197