fix overwriting modes in project like IJulia

Current IJulia custom.js works by setting the right option of CodeCell , but do not have (yet) kernelspec. 

This leads to master not using the mode set by IJulia, and defaulting to 'null' which remove highlight.

This seem to do the trick by leaving default highliht mode and/or respecting metadata.language which is set by  alot of projects. 

I don't think we should do this. I put in a fallback to `null` quite deliberately so that syntax highlighting would get turned off if you switch to a kernel where any syntax highlighting would be misleading.