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've come to a conclusion that docstrings are only *necessary* for @cmd type of functions.

In here, the problem is that although the function name would be pretty self explanatory, it depends on a variable (self.count) which is not right away obvious without looking at the function name. That is why I believe a comment is appropriate here in order to reduce future confusion.

Also, while we are at it the name `msg_formatter` might not be the best one, especially if one wants to read code as poetry. I believe it would be way better to have something like `format_msg(time_unit)`.