Add support for jupytext notebooks

[nthiery]

Like many, I am using more and more jupytext to author my notebooks. It would be great if nbviewer would display them as jupyter notebooks instead of as plain markdown files.

Example: open 2021-02-15-SageOscarDays.md from:

https://nbviewer.jupyter.org/github/nthiery/shared-sage-notebooks/tree/master/

Presumably this could take the following steps:

• Have nbconvert support jupytext notebook by using jupyptext.read
• Having a hook in nbviewer to detect from the metadata that a markdown file is actually a jupytext notebook

See:

• https://github.com/mwouts/jupytext/issues/743
• https://github.com/jupyter/nbconvert/issues/1521

Thanks in advance!