prefer marked to pandoc for markdown2html

install with `npm install -g marked`

for better consistency with the live notebook

This is more a proof of concept, I'm not sure if we should actually go this way.

Still TODO: to really match the notebook, we would need to strip/re-add math around marked, but the way we do this in js is already wrong in several cases (most notably within code blocks). This is one of the things pandoc's parsing handles better than marked.

ping @ahmadia, who was interested in this.