Allow tex2jax options to be passed into math plugin.
Allow tex2jax options to be passed into math plugin.

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.
      });

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