Add !countdown command for countdown timers

Usage: !countdown <time|left|stop> - Set the time with !countdown #d
(for days), #m (for minutes), etc.  The "left" subcommand will display
the time left if there is a timer, the "stop" subcommand will stop the
timer if there is a timer.  Every timer runs in its own thread and every
user will have their own timer (when they create one).