Show subtitles where available

[benrito]

Regarding #83,

@gridinoc is enthusiastic about using subtitles when available, so users can do that Facebook-video style that works well with no volume.

Consider implications for #79

[benrito]

@antiboredom

[benrito]

@Laurian

[benrito]

This would be done, per @Laurian suggestion, by including text track events in the Hyperaudio > Popcorn JSON.