handle MathJax
handle MathJax

* include resource file containing current MathJax URL with
  `data-external="1"` to prevent incorporation of the lib into
  the generated html
* use `--mathjax`, i.e. Pandoc should transform tex math into
  MathJax format
* point MathJax URL to local dummy file to prevent incorporation
  of the lib into the generated html

no need to use sed anymore :)

cagix/pandoc-lecture