No start override

[ederag]

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

fix #560 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. 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:

[ederag]

0cffe9c reverted to allow testing only 😉

[mwilck]

Thanks, this works better than before.

[GeraldJansen]

Tested this also with xfce-hamster-plugin and "everything" (well, everything I tested) seems to be working fine now.

[ederag]

Thanks !
Removed the revert, prior to merging.