Javascript builds

[choldgraf]

We also need to support dependencies for Javascript (maybe including things that require a function call after installing, e.g. some of the jupyter extensions?). Discuss implementations etc here!

Note that our guide on repo2docker already starts out by showing how to create a builder with npm. Perhaps we can create a javascript build in parallel with finishing that document.

[choldgraf]

just a thought: should we just capture this with postBuild?

[manics]

If this is purely for jupyter extensions it could be part of #868 ?

[consideRatio]

Hey, starting out some triaging - this is the oldest issue open in repo2docker and seems to me like something not worth supporting without a stronger pull for it.

I'll go for a close at this point