Added simple option, please review

[Ingo Lohmar]

Hi all,

please find below a super-simple patch to fix calendar day header
fontification.  Currently, Sat/Sun are fontified as weekend days,
independent of calendar-week-start-day.  Instead of depending on that
variable, it's better to introduce a separate one right away: The
leftmost day of the display need not be the 1st/2nd or whatever weekend
day.

I'd be happy to hear about any formatting/changelog/other issues, and
will try to correct them.  If one or two people find the patch ok, I
will commit.

Thanks
Ingo