Consider letting reveal do the code highlighting or dropping it entirely

[dabrahams]

Aside from being at the root of #1 not being implemented, having jekyll/rouge put the highlights in the HTML makes it much harder to consider scanning code examples with our own plugin, e.g. to look for special annotations that drive animations. It also injects some extra <div>s that cause this CSS to be needed and other kinds of customization to be impossible from markdown.

It would allow us to use reveal's marking when only line-by-line marks are needed the usual case, and save the reveal-mark-plugin for things that need sub-line marking.

I'm not sure why I decided to let jekyll do this job; maybe it was a misguided desire to avoid client-side processing.