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

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

Open/close with Control+Shift+f
Find words inside SPAN
Don't duplicate findings

Added programatic support for custom key bindings with optional descriptions to be added to the help screen

Add an option to display advice on whether the current pace of the
presentation is on track for the right timing (shown as green), and if
not, whether the presenter should speed up (shown as red) or has the
luxury of slowing down (blue).

The pacing timer can be enabled by configuring by the `defaultTiming`
parameter in the `Reveal` configuration block, which specifies the
number of seconds per slide.  120 can be a reasonable rule of thumb.
Timings can also be given per slide `<section>` by setting the
`data-timing` attribute.  Both values are in numbers of seconds.

When the option is enabled, clicking on the timers will reset the timer
to the beginning of the current slide, i.e. as if pacing was perfectly
on track, not to zero as if the presentation had just begun.

and make muting work for negative values

This allows us to use different colours for different timers,
which will be useful when we add a pacing timer.

This will allow us to reuse the display code for displaying
an additional pacing timer.

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