Feat: support script files

[maartenbreddels]

Depends on #309 (alternative to #328)

Now we can do:

$ voila tests/notebooks/print.jl
$ voila tests/notebooks/print.py

It works by injecting the script code into an in-memory single-cell notebook, which means the last expression is printed out. If for some reasons one needs more output, you can use display(expression), or use jupytext (which should also be supported with this PR).

[maartenbreddels]

Example usage:

voila --VoilaConfiguration.extension_language_mapping='{".py": "python", ".xcpp": "C++11"}' \
  --VoilaConfiguration.language_kernel_mapping='{"python": "xpython"}' tests/notebooks

Now you can access:

• http://localhost:8866/voila/render/print.py (using xeus-python kernel)
• http://localhost:8866/voila/render/print.xcpp (uses xeus-cling's C++11 kernel)

The tree handler and the voila handler (both the standalone and server extension) will serve and show .ipynb and all the extensions listed in this configuration option.

The final step would be to document this

We might also want an extension_kernel_mapping configuration, to go directly from .py -> <kernel_name>, but maybe that is overkill.

[jtpio]

We might also want an extension_kernel_mapping configuration, to go directly from .py -> <kernel_name>, but maybe that is overkill.

That would remove one level of indirection, but I guess having both extension_language_mapping and language_kernel_mapping gives enough flexibility. Also we would need to handle conflicts or precedence if the 3 options are used together (in a conf file for example).

[jtpio]

We can also consider adding aliases for these options if they tend to be used often.

[jtpio]

Maybe later after adding support for the progressive rendering (#133) there could be more than one cell created, from cell delimiters such as %% or # In[1]:.

[maartenbreddels]

Yes, this could again be a configurable, mapping from file extension to regex. I'd say not now right?

[jtpio]

Indeed let's do that later (we can open an issue to track it).