add nbextensions

serves files in `IPYTHONDIR/nbextensions` at `/nbextensions`.

This is a location for users / devs to drop-in frontend customization as js modules, css, etc.

The model is very much like the extensions dir (hence the name) - it's a location made available, but not loaded by default. You can load things from there with require, etc.

It's a configurable search path, just like static. This way we can define a system-wide location at a later point (/usr/share/ipython/nbextensions?) and still have user installs.