Allow rendering plain text output from Jupyter kernel execution
See discussion at https://community.manuscripts.io/t/sympy-module-not-found-error/111
IMO if no image output is available for a figure with a code block, we should be rendering the figure in textual form. I had envisaged a separate node type for this kind of code expression outputs, but it does seem reasonable to say that there are cases where you would want to crossreference such code execution output even if it is not visually drawn into a bitmap or vector graphic form (i.e. that it is a figure-like result).
How this kind of material gets exported is a different matter, and we may need to tackle that separately.