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>
Can you update this comment to be a bit more descriptive? Also, a comment should have a space between `#` and the comment message itself.