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.
Disclaimer: I don't know anything about ruby and very little about slim

Without quotes around the #{...} it gets through as true (as opposed to 'true') which is the proper javascript boolean. You can see strings later in the file:

    -         transition: Reveal.getQueryHash().transition || 'default', // default/cube/page/concave/zoom/linear/fade/none
    +         transition: '#{attr 'revealjs_transition', 'default'}', // default/cube/page/concave/zoom/linear/fade/none

Or maybe you mean that I should use Ruby true / false keywords instead of strings? If so I'll happily switch. I didn't even test to be honest.