Add hooks for overriding default cue styling.
Add hooks for overriding default cue styling.

To assist players in complyinig with US accessibility regulations,
add a WebVTTSet dict the caller can poke user-defined preferences
into for font family, size, color, etc.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1167492

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@mozilla.com>

mozilla/vtt.js,mozilla/vtt.js