Added support for exporting Reveal.js slides.

Pretty much as indicated.  Reveal.js is a popular html5 slideshow template and it uses a slightly different format, making it difficult to use it with plugins or templates alone.

* 'incremental' elements have `fragment` tags on the actual elements, not the parent.
* Reveal.js supports 'stacks' of vertical slides as embedded section elements.

These changes and a couple other are a part of this pull request.