Add a nxfvars execute command

[Gregor Sturm]

Add a nxfvars execute command that calls jupytext/nbconvert to execute and convert a notebook to html.

This should respect jupyter-book compatible tags to

• hide inputs
• hide outputs
• hide both

Optionally, support a nicer template than the default nbconvert one:
https://blog.jupyter.org/the-templating-system-of-nbconvert-6-47ea781eacd2

[Gregor Sturm]

nxfvars execute is now implemented, but it does not yet respect the tags to hide cells.