Pizarra notebook extension

[Kiko]

Hi all,

After giving several courses during May I thought it would be cool to
have something to paint over the notebook contents in order to explain
some concepts of the code, output, etc, from the notebook itself.

This is why I've created a new notebook extension called Pizarra (the
spanish word for blackboard).

The repo is here: https://github.com/kikocorreoso/pizarra-nb
In the README you can see the extension in action. Also, you can
launch a binder to test the extension:
https://mybinder.org/v2/gh/kikocorreoso/pizarra-nb/master

I think the extension is usable (only tested on Linux, latest notebook
version and latest Firefox version) but it needs a rewrite. At this
stage is just a proof of concept and I would like to add some more
functionality (transparency for the brush, ability to paint
rectangles, ability to save the resulting canvas as an image,
possibility to add the result in a cell below the active cell,
possibility to add arrows,...).

Please, could you test it and tell me if you think it is useful?

You could use the GH issues to ask for potential improvements that
could help you when you are teaching with the notebook:
https://github.com/kikocorreoso/pizarra-nb/issues

Thanks an advance.

Best regards.