JupyterLab support

[kzielnicki]

JupyterLab is the next-generation frontend for Jupyter, and has a more modern extension framework. This should be possible to achieve better frontend integration for Nodebook.

See also Issue #12

[kzielnicki]

This may be a little bit tricky because Jupyterlab maintains a better MVC separation, which helps a lot with proper extensibility but makes it trickier to do all the the monkey patching I was doing before in one place. The most straightforward way forward is probably to patch codecell to do a similar cell magic decoration to what I was doing before, but still TBD on whether I can access the cell parent from within the codecell and inject this into the code block.

Another route that may be better would be to use a custom kernel and rely on metadata to establish cell order. See, eg, this recent Jupyter PR which adds a cell id metadata to the kernel payload: jupyterlab/jupyterlab#5033 and the reactive kernel project here: https://github.com/jupytercalpoly/reactive-kernel