revealjs: speaker notes support in slim template + docs

I tested it and it works.

I included class `aside` and `speaker` because `aside` is the official name of the html5 tag in reveal.js (and speaker so it is consistent with deck.js).

I took the liberty of copying the `EXAMPLES` section of deckjs and adapting it to reveal.js. I didn't document feature I didn't see working so even though I know that the slim templates probably does more I haven't wrote it down.

By the way, thanks @mojavelinux for asciidoctor. I never figured out the obscure "templating language" used in asciidoc. So I never contributed something worth anything. The layout with several files with clear names and the powerful variables accessible inside them really makes modifying backends easy (and definitely way more readable). Thanks!