Add jump to time, clean up scoping

* Elements with data-click-to-time=true will jump to the
  corresponding time in the video when a text cue is clicked
  on
* Text cues will have the data-start-time attribute set for
  CSS styling
* Tracks without kind=subtitles will be ignored; only the
  first text track will be used
* Demo displays time codes for each cue
* Can now be embedded multiple times on a page