Notebook: let user code output HTML to render.

[switham]

Give the user's Python code a function to output HTML to be rendered in the output window (interspersed with typewriter-style stdout and stderr).

Maybe follow the Python 3 print function API.

Maybe make this safer (& easier though nonstandard) by making available (a subset of) the procedures I use internally to generate HTML elements.

This tangles a little with gradual "streaming" output if I try to force that HTML elements are always closed properly. But... I don't think badly tangled.