Countdown: Added option to set countdown delay.

 * There are two options to call countdown now:
	- with user defined delay: !countdown <timer> <delay> <reminder_msg> which sets frequency of "T-{x}" output
	  to <delay> seconds.
	- without user defined delay: <delay> is automatically set to 60s.
 * Moved countdown output, so it doesn't say T-0 when countdown is finished.
 * When countdown is finished, returns a message "It's happening! <reminder_msg>".

Signed-off-by: Jakub Novak <jakub.novak@gmail.com>