Add background support

If the first element of the slide is an image, make it the background
for the slide and scale to the maximum.
Special classes for image slides

The markdown syntax for an image has a field for alt text. Since we turn
the image into a background, there’s nowhere we can put the alt text.
But we can (ab)use it to assign a special class to the slide, which then
can be used by the presentation author to style text of the slide.