Classic notebook extensions

[gnestor]

Create a wrapper for the labextensions that will allow classic notebook to consume them with little to no modification.

[gnestor]

@rgbkrk recommends using rollup:

rollup main.js --format umd --name "myBundle" --output bundle.js

[rgbkrk]

Are you going to include the data resource one here or build it in to classic directly?

[gnestor]

I would like to keep all the renderers in this repo, so once I figure out how to wrap these lab extensions into classic nbextensions, then I will add the data resource renderer here. Whether it will use react-virtualized or phosphor's data-grid renderer is a different question (but I don't see why both can't be provided by this repo).