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.
Merge pull request #54 from obilodeau/revealjs/expose-config

revealjs: expose all configuration through attributes (w/ doc)
Merge pull request #56 from obilodeau/revealjs/step-option-or-role

revealjs: fragments support with step option or role
prefer step option over role for reveal.js backend
use symbol as argument to option? method
fix path to print stylesheet