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>
I believe we agreed we would only add docstrings to @cmd type functions. Also the function name is pretty self explanatory.