Support 24h clock

[galan]

Hi, when trying to use the timer using eg. ti 18:50 Hello it states that it is an incorrect request. Only am/pm seems to be supported.

[vanrein]

Yes, am/pm are really local choices, and it should be trivial to add syntax without AM or PM added. It would even offer a shorthand that allows dropping AM.

[brunetton]

Fixed in #20