Implement the auto install of extensions

[SimonBiggs]

Details are within the following pull request

jupyter/notebook#3116

[SimonBiggs]

https://groups.google.com/d/msg/jupyter/eTjvZdM7VSc/aaakoaD6CQAJ

[SimonBiggs]

@robmarkcole essentially I need to implement this enhancement which will make it so the jupyterlab extension will self install when "pip install scriptedforms" is run. Once I've done that I'll release 0.9.2dev0. I'll get you to put it through the ultimate acid test :) and then once we're happy with it I'll release 0.9.2.

[SimonBiggs]

I do suspect unfortunately that given extension installation will still require yarn package downloading you might have a similar trouble to what you had previously.

I do believe there might be a simple solution to that. But let's cross that bridge when it comes.