Add scripting to apply ableplayer on all video/audio elements.

[joedolson]

• Parse all video/audio elements and add the required data-able-player and id attributes if not already present.
• Following settings, add transcripts if captions present.
• Parse MediaElements playlist and apply AblePlayer playlist attributes.

There may be other things to consider; will discover while exploring.

[joedolson]

Probably also need to dequeue MediaElements scripting and styles if this option enabled, or at least remove the MediaElement attachment from media players that don't have an exception class.

[joedolson]

Second task identified a bug in Able Player, noted in ableplayer/ableplayer#618. Handled in plugin by auto-assigning the default value if omitted.