JupyterLab 3.0 support

[yuvipanda]

Currently this doesn't work with JupyterLab 3.0. It should.

[bollwyvl]

I can look at this, unless somebody else was going to?

I think a couple key things got merged upstream (like connecting to the JSON message bus) which start to make this very interesting for e.g. whiteboards, broadcasting scroll/cursor position in a notebook, etc. so it probably makes sense to figure out a way to consume (and provide attribution) for the upstream code more directly.

[yuvipanda]

@bollwyvl that sounds great! Will appreciate anything you can do :) Making a JupyterLab 3.0x compatible release - even if on npm - would definitely unblock me at least :D