Closes #2026.

This does not change the default behavior of the math plugin,
but it allows $ delimiters to be disabled (which is usually
what you want, since otherwise normal uses of $ for currency get
treated as math delimiters).

To use:

    Reveal.initialize({
      math: {
        tex2jax: { inlineMath: [['\\(','\\)']],
                   skipTags: ['script','noscript','style',
                              'textarea','pre'] }
            },
      etc.
      });

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.