Add results capture

[Klafyvel]

Following #15, I would like to be able to capture the evaluation results to include them in the file that generated them, i.e. transform the virtual text into real text. For images, it would mean saving them somewhere and putting a link. We can do smooth things and add some markup for three kind of files:

• markdown: results as code blocks, images as markdown images
• norg: same, but with norg syntax
• .jl files: assume they are Litterate.jl notebooks, and do as would be done in markdown in Julia comments.