The current pacing timer operates on the assumption that there is
a default amount of time to be allocated to each slide, and that
individual slides can deviate from that default by specifying their
own data-timing attribute.

This patch introduces an alternate pacing method: by specifying
the totalTime configuration option, the presenter can set the total
time available to present. The pacing timer will then continue to
allocate the exact pacing time for slides that do have data-timing
set, as before. However, rather than applying the defaultTiming
constant to all others, it will

- Add up the time already allocated to slides via data-timing;
- subtract that from totalTime;
- divide the difference by the number of slides without data-timing set;
- apply the thus-calculated average to those slides.

totalTime has no default, and if both defaultTiming and totalTime are
set, totalTime wins. This preserves backward compatibility: if a
presenter has set defaultTiming and updates reveal.js, totalTime will
be null and defaultTiming is still applied to all slides without a
data-timing attribute. The presenter can then switch to the automatic
calculation, if desired, by setting a value for totalTime.

Allows lazy-loaded (i.e. data-src) iframes to be preloaded when they
come within the viewDistance, rather than once they're visible.

Resolves #2211

The Install was waaaay at the bottom, which didn't make sense to scroll through all the detailed documentation, just to find how to get started with an install.

Fixed broken link to markdown options doc

- Use `single backticks` consistently for inline code snippets.
- Add double linefeeds before level 2 headings.
- Add a single linefeed after each heading.
- Minor docs edits.