Protection against double-bookings

[tobixen]

Ref #29

The "normal" user behaviour would be to first check the calendar to see that there aren't any conflicting events on the calendar, and then add the new event - however, it may be more efficient to specify something like --check-conflict. If the new event overlaps with some existing event, instead of adding it to the calendar, an error is thrown informing about the double-booking. With a --interactive option the user would interactively be asked to (1) leave calendar as it is, (2) add new event regardless of the overlap, (3) cancel the existing event and add the new.