optionally display pacing advice based on slide timings
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.