Implemented the #deadline=yyyy-mm-dd tag.

The Task.carryover(date) method is used to carry tags over from
one day to another. The TaskFile.__process() method uses it,
iteratively carrying-over task from the last run date until today.

Additionally:
(1) Shifted helptext to a separate file.
(2) Added the `--nosave` option for convenience of testing.
(3) #essential works like #deadline, with no time limit.
(4) Fixed bug in TaskGroup.task_list() where the actual list
    reference was being returned. In conjuction with
    TaskGroup.task_remove(), this would skip items in the loop.