Add a postprocessor to customize nbconvert's html output css classes for...

... Javascript highlighters

This is a followup to https://github.com/ipython/ipython/pull/4682.

I placed the code in a "python" subdirectory of the "publishing" directory because it is related to publishing nbconvert's output on blogs.


I'm now a bit unsure if this code fits in this repository (it is Python, not JS), but I submit this pull request to start the discussion :)

Daniel