Lots of little updates.

(1) Table Headings are no longer fixed, but based on task-groups.
(2) Words appearing after a "//" are considered to be the result.
(3) prettytable textwrap uses the largest word as minimum column width.
(4) New "exceeded" status tag introduced.
(5) Performance index can now be accessed through a separate "report" operation.
Updated behaviour of Periodic Tasks & Deadlines

Converted task.__nostatus to task.__tagfilter to remove both status & periodic tags.
Resolution of relative deadlines is deferred till instantiation of periodic tasks.
Added --nodeadline option to assume +deadline=none if not specified (instead of today).
Tasks inherit the --nodeadline option, for tabulation purposes.

Ok, the code is getting are getting ugly again :(
I don't like how the taskfile passes down the date & nodeadline settings.