Allow override of HTML escaping in code blocks

Hi Hakim & al,

first off: thanks for your hard work on reveal.js, it has become my main presentation tool.

I noticed PR #432 now escapes all HTML in code blocks. This is unfortunate for me, since I often use HTML and fragments to highlight portions of code I'm talking about. (I even wrote a small plugin for this - you can see it in action [here](http://ericweikl.github.io/revealjs-charred/#/2).) With the current version, this doesn't work anymore and prints the escaped HTML instead.

The attached PR allows you to prevent HTML escaping by supplying an attribute `data-noescape`, similar to the `data-trim` attribute. By default, HTML will remain escaped, which makes sense IMO.