Stop abusing the HTML element ID

Instead, use the `data-` prefix to store our own information.
Allow background images to scale to fill the slide

The default is to scale the background to cover the slide. If you
specify a slide style os `background-fit` then the background will
be scaled to the largest size that completely fits on-slide.

Fixes #181
Merge pull request #210 from binford2k/ticket/209/form_id_handling

Stop abusing the HTML element ID
Merge pull request #211 from binford2k/ticket/181/scale_large_images

Allow background images to scale to fill the slide