Nbconvert and unicode variable names

Select the highlighting lexer for pygments based on the notebook
kernelspec metadata from codemirror.
This fixes https://github.com/ipython/ipython/issues/6618
1. We should add a key for `pygments_lexer`, rather than hoping that it will have the same name as the codemirror mode.
2. The code will need to handle noteboooks where the metadata doesn't have a kernelspec (i.e. all notebook saved before IPython 3), and probably notebooks where the kernelspec object doesn't have a pygments_lexer key.
3. You shouldn't assume that codemirror modes have separate name and version fields - that's not what the spec says. They can be a single string. But this is irrelevant here, because of 1.