Break records across prose+code block boundaries and clean Jupyter notebook in/out furniture

[jonathansick]

To get cleaner snippet highlighting from content that includes code, we might want to break records down to just prose and just code blocks and then add a flag to the record indicating whether it's code or not. This could let us style matches to code blocks using a monospace font, and hopefully even retain line formatting.

As well, we probably want to strip the In/Out furniture that Jupyter adds around code blocks and their outputs.