1465 Add "background div" to allow HTML backgrounds

[nickshine]

This feature would be a nice addition, but maybe using <template> for the html that is acting as the background to avoid having to put style="visibility: hidden;" explicitly.

[nickshine]

To do:

• implement in to latest reveal.js code
• properly document examples as it is a little less straight-forward than the data-background-image or data-background-video types.

[nickshine]

hakimel#1465

[cujarrett]

I’d lean towards documenting it but don’t feel strongly either way.

[LongLiveCHIEF]

I've started to wonder how much styling the core of reveal should have in 2.0, and whether or not all "theme" styling should be plugins based on hooks the core implements.

"Theme" being anything that isn't basic css required to handle presentation functionality.

For now, as I've been reviewing, most styling ones that seem basic, I put on the "planned" list, and figure they can be broken out if we make all "theme" based stuff implementable via plugins.