cdn.mathjax.org is shutting down: https://www.mathjax.org/cdn-shutting-down/
This changes the default MathJax URL in the math plugin, as well as
references in README.md and test/examples/math.html
Just using $ as delimiter in markdown document fails since the markdown
parser unknown to the dollar syntax will try to interpret underscores.
Putting the $ delimented formula in backticks will cause the markdown
parser to put the tex-code with the $ delimiters into a code block.
The texcode will then be unchanged. This patch allows for mathJax to
interpret and automagically display the tex-formulas.
