add javascript support

[basnijholt]

With this we can (for example) include HoloViews, Bokeh, or plotly plots in the docs.

This is already used in https://adaptive.readthedocs.io

[akhmerov]

Thanks!

There are really two parts to this PR:

• How to include JS output if it ever appears. This is rather uncontroversial, and as far as I can tell looks good.
• Making JS output the highest priority after widget type. This is user-controllable, so it cannot lead to dramatic failures (the users may specify a different priority if the default one doesn't suit their needs). On the other hand, the default behavior should be reasonable. I believe the main use for javascript output is pre-jupyterlab libraries injecting their javascript into the notebook after one calls something like init_notebook. This justifies making JS a high priority. This also matches the current behavior of nbconvert's HTML exporter.

Therefore I suggest to merge as is, but I'd like @SylvainCorlay's opinion—he dealt with JS in notebooks more than I did.

[SylvainCorlay]

I believe the main use for javascript output is pre-jupyterlab libraries injecting their javascript into the notebook after one calls something like init_notebook.

Yeah, it is unfortunate that some packages like bokeh use javascript injection instead of relying on notebook extensions.

Plotly plots should already work with jupyter-sphinx without javascript output since plotly moved to using jupyter interactive widgets.

JavaScript injection pauses other issues. I am fine with adding it here since we can assume that the documentation content is trusted however I don't have an opinion on the precedence order.