Add Jupyter Notebook output

[avsm]

This is a WIP; creating a PR to make it easy for @jonludlam and me to push to our forks

• detect and reject multiple environments in one notebook (or figure out how to make them work in Jupyter)

[avsm]

Some quality of life improvements: supports Markdown sections and raw cells now

[avsm]

Noticed a problem that causes divergence between mdx evaluation and Jupyter. The latter doesn't support multiple environments, so having two OCaml environments will cause them to be silently merged into one with the PR.

[craigfe]

I have a rebased form of this PR at CraigFe/jupyter if you do decide to go ahead with this.

[avsm]

thanks, I've pushed your rebase here. @NathanReb we could go ahead with merging this; the only restriction it would be nice to have before that is to ensure that there is only one environment per notebook.

[craigfe]

From playing around with this PR, I noticed that it doesn't yet support file includes. I have a fairly hacky patch for that here, but it probably warrants a cleaner implementation (that doesn't just fail for a large number of block types).

[avsm]

Could we get this merged as an experimental feature @gpetiot, so that @craigfe's variant above can be tracked? It's already useful as-is despite the limitations.

[avsm]

rebased to current trunk