We would like to, at a minimum, allow for code outputs such as plots and tables to
have a label and caption. Then also e.g. the other options allowed by docutils figures.

Using pasting and gluing (see b46c2f1 and #70) actually gives us a lot more flexibility in this respect, to add these options with markdown cell text.
It is basically an implementation of the Model-View-Controller pattern that I have banged on about before, so I would rather use this approach.

But for 'simpler' use cases we may also want to allow for these to be set in the metadata of the code cell, under a suitable key name:

{
  "output_format": {
    "label": "fig:myfig",
    "caption": "blah blah blah",
    "width": 400
}