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.
Aha, I see what you are saying. Yes, that makes sense now. I think we'll still need to include a helper function to prevent empty values (which would break the JavaScript), but we can do that in a follow-up PR.