Jn/countdown

Countdown: Added countdown command.

* !countdown <time> <task>
* Countdown timer from 'n' (time in minutes set by user) to 0 for a task (text set by user).

Signed-off-by: Jakub Novak <jakub.novak@gmail.com>
Well, I could certainly live without it but yeah, a docstring would be nice here.

The thing is that I cannot really think of what kind of a docstring would you put here?