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>
@mrshu: How exactly do you imagine to abstract this into function? Because unless he uses some kind of higher-order functions / macros, I don't see a way.