Support the Jupyter messaging protocol

[nathancarter]

If pythontex supported the Jupyter messaging protocol then it would automatically support all these languages. That would be an efficient way to add a ton of new languages to pythontex all at once.

[gpoore]

Adding Jupyter kernel support has been on the long-term plan for a few years now as part of a complete rewrite of the code execution core, but unfortunately that project is moving very slowly.

[nathancarter]

A decent subset of the desired functionality is possible within the existing version. See #149.