revealjs: speaker notes support in slim template + docs
revealjs: added more doc: data-background usage
revealjs: expose all configuration through attributes (w/ doc)

Took the liberty to update available configuration parameters up to
reveal.js' current master (which is stable).
revealjs: document revealjs_theme and revealjs_customtheme attributes
revealjs: fragments support with step option or role

Includes documentation. It was divided to make things clearer.
prefer step option over role for reveal.js backend
use symbol as argument to option? method
fix path to print stylesheet