fix the code highlighting by add missing pandoc defines

This code works on my machine. However, I don't think the way that mkbok
produce the output is right. I think what we should do is to provide our
own latex template, not sedding the output of pandoc here and there.

Maybe I'm wrong. ;-)
Merge pull request #1 from grissiom/fix-code-highlight

fix the code highlighting by add missing pandoc defines