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>
Should this message change when `self.minutes` is zero? Something like '{1} is in progress' or whatever you like?