add js_extensions_path

serves files in `IPYTHONDIR/js_extensions` at `/js_extensions`.

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/js_extensions?) and still have user installs.
s/js_extensions/nb_extensions
install offline mathjax to nb_extensions instead of profile
serve local mathjax from nb_extensions
s/nb_extensions/nbextensions
alternate interpretation of mathjax install dir

parent, instead of full path
create static/custom in skeleton profiles
create [nb]extensions dirs in skeleton IPYTHONDIR
update IPYTHONDIR readme

it wasn't accurate
Merge pull request #4277 from minrk/jsext

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