Add date to first slide

[wander]

In slides mode, add keyword to add a date, e.g.:
#date 2012-08-21

Font size should be smaller than author name.

[weis]

Which date? The current date or the date when the slides have been modified?

[wander]

Date to be entered by user. If no date is specified, fall back to current day.

[wander]

I'm withdrawing this request in favor of #7.