[WIP] try to fix custom.js

Should fix #246 (partially) but work in progress. 

It is a due to a complex interaction between codemirror 4, that is now asynchronous, the kernelspecs of IPython master and a few other things. @dbartow might want to try that. 

Notebook will work but you won't have highliting though. 

https://github.com/ipython/ipython/pull/6850 should fix that.