Classes in markdown element

When you are writing a presentation in markdown, you lose all the reveal attributes on generated html elements like paragraph or list items with fragment attributes.

This allows to specify right in your markdown presentation the reveal attributes you want to see in the *enclosing* html element generated from that markdown section.  
See the https://github.com/VonC/reveal.js/blob/classes_in_markdown/test/test-element-attributes-markdown.html for examples.

It should also solves #431.

It is independent from my other proposal #640 (Attributes in markdown) which adds data-transition or data-background reveal attributes to the `<slide>` elements.
I intend to combine both to get the full benefit of reveal slide effect and element effects in my markdown presentations.