cal: added -w|--week for showing week numbers

Added week numbers both in ISO-6801 and North America numbering.
The mode is determined by first day of the week, Monday
for ISO and Sunday for North America mode.

ISO week numbers are defined as the first Thursday being part of week 1.
The North America numbering is defined, at least by gcal, as first Sunday
being in the first week.

Signed-off-by: Tommi Kyntola <kynde@iki.fi>
cal: various tests for week numbers

Current tests were replicated with week numbers
and some additional tests for week numbering corner cases
were added, too.

Signed-off-by: Tommi Kyntola <kynde@iki.fi>