Suggestion: Support asciinema recordings

[tarruda]

Currently, we can do that manually by pressing ctrl+z and invoking asciinema play and then going back to the presentation with fg.

It would be great if we could put something like this in the slide:

[asciinema:80:24:./demo.json]

This tag would create a rectangle of 80 columns and 24 lines to play the recording. Clearly this involves some kind of terminal emulation, which could be achieved more easily with something like libvterm