Defined exposing SCTE 27 subtitles with DataCue

Changes to use DataCue for exposing SCTE 27 subtitles. This involves changes in: item 3 - setting kind, item 5c - subtitle cues
Yes, I think when the browser decides to follow the CEA708 or SCTE27 rendering spec, it should also expose appropriate parsed cues. Whether they be CEA708Cue, SCTE27Cue, or a conversion to VTTCue doesn't matter. What matters is that the browser has implemented rendering and thus knows the structure of the data and thus can expose a JS API for it.