revealjs: expose all configuration through attributes (w/ doc)

Also took the liberty to update available configuration parameters up to reveal.js' current master (which is stable).

You might get conflicts with the other pending pull request I made earlier but it'll only be because they both touched the README inserting text at similar locations. `== Settings` should go before `== Example` (to follow deck.js' doc). Sorry about that.
> History is nice when designing slides (so you can refresh and be at the right spot) but I find them quite annoying when I view presentations made by others online.

I actually agree with you. Perhaps in the README where this property is documented we could add an admonition note with this reasoning so at least users understand the default setting is intentional on our part.