A newly added periodic task may be added to today's list too.

(1) The Date constructor can now recognize datetime.date instances.
(2) The tabulation headings have been removed because I didn't like how they looked.
(3) Date instances (instead of datetime.date) are passed to Task instance methods.
(4) The action command-line option now has a more flexible format.
(5) Sorting of Tasks within a TaskGroup is now case-insensitive.