captions

We currently have a 'caption-side' property for manipulating table captions.
They can be moved above or below the table without affecting the markup at all.

I think the same kind of control should be available for figure captions, source
code snippet captions, etc. We could turn them all into tables, but that doesn't
seem appropriate: tables carry a lot of other baggage besides the ability to
manipulate caption positions.

~fantasai