fixed showing speaker's view with timings/pacing while serving the presentation from the file system

Ref. #1856, #2006, #2045.

This is a more open approach to allow customisation of all MathJax options instead of select options only.

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.
      });

Block dimension calculation is not accurate, so zoom is made on mouse coordinates
Change default modifier to ctrl - alt don't work on Linux
New parameter zoomLevel: default 2