Date highlights

[helgur]

It would be really useful if you could pass an array property which contains a list of dates the calendar highlights. A concrete use case would be if you're using the date picker in an administration screen to select what posts to edit, and it defaults to display one month old posts (but no posts older than that), instead of guessing when in the past there are older posts you could edit, if the date picker has highlights in the calendar, you'd know which dates have posts.