Support markdown content

[fcollonval]

markdown. Being able to opt into some markdown (which might include
commandlinker links) for the step bodies would open a lot of opportunities

These last three kinda quack like "middleware," which could be registered
by plugins and opted into in a tour definition in the settings/command
API... As these would not be part of the official types, they'd need a new
place in the schema, e.g. "extras" or "transformers," e.g. {"markdown":
true, "xpath": true, "commands": true} at the simplest, though they might
need configuration...

Originally posted by @bollwyvl in #17 (comment)

[bollwyvl]

Thanks for splitting this out.

This is kind of a challenge, as likely a custom step component would be needed, with the Lab typesetting object handed down into its props, with some special sauce on componentDidMount, portals, or whatever. Ick. But would still feel really good. I hacked this with even ickier setTimeouts on @deathbeds/jupyterlab-rjsf and don't like it all, but in that case, there are markdownable fields all over (title, description), and i didn't want to make a full theme yet...