Permissive parsing of DELTA

[madduck]

I often enter the DELTA using khal add with a leading plus, e.g. because it's how stuff works in TaskWarrior, among others. So: khal add tomorrow 10:00 +90m Test, but this generates an event with title "+90m Test".

My suggestion would be to swallow a leading + in what might be a DELTA value, and try both.

Happy to try to submit a patch.

[WhyNotHugo]

I think that this sounds pretty sensible.