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>
@JakubNvk ok, cool

@pepol what do you think about this? Should there be a command which will actually set the countdown delay? Should it be part of the !countdown command? I quite like the latter actually.