Countdown: Reworked countdown.

 * Removed the option to set delay time.
 * Countdown now automatically changes delay time depending
   on time left.
 * Fixed a typo in docstring.

Signed-off-by: Jakub Novak <jakub.novak@gmail.com>
Well @mrshu, I think that it's necessary so we don't announce it's T-0.7min for example, it's basically formatting of output message (minutes, seconds...).