Add test for nbconvert's html rendering with mistune

[mpacer]

Currently this writes out the notebook as a notebook and stops there to be explored by hand & eye.

We also have nbconvert which has an html exporter that uses mistune to convert the markdown to html.

Mistune has different behaviour from marked so it should be checked separately.

[jzf2101]

@mpacer this issue seems sprint friendly, confirm?

[mpacer]

I wouldn't say that doing any of this is per se sprint friendly. It is new contributor friendly in the context of a extended mentor/mentee project. The problem is that understanding this well enough to do it correctly will require ramping up a lot of background knowledge.