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

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.

When a slide has several fragments it could be convenient to define a note for each of them. In this case we need to show only this specific note defined in a fragment and not others. General note of a slide shouldn't be also shown, as a more specific one should have greater relevance in this case.

Closes #1392.

It would be nice if we could define a custom notes.html file.

Actually, I'm used to compile my js files before releasing my app and, using selectors like `script[src$="notes.js"]`, doesn't work :(

So, what do you think about it?