No start override

Created by: ederag

This is #491 (closed) again, supplemented with "if the parsed fact has no start, use now". Should be better than #492,

fix #560 (closed) while keeping full backward compatibility with existing clients.

@jmberg the 10:00 meeting activity can be entered either by tricking (e.g. prepending a character: .10:00 meeting), or (that's new) by using double dashes -- as explicit missing times (see help). Here That would be -- -- 10:00 meeting.

@mwilck Thanks for your test case in #491 (closed). Now the time would not be ignored, so in some sense #492 was not, while this one is fully backward compatible. If a client wants "activity without range" (not recommended at all), it could just prepend -- -- to the string: