created MPEG-2 TS cue table based on WebM cue table
added mpeg-2 ts text track if PID == 0x02
Merge branch 'upstream'

This adds changes from Bob from:
https://github.com/w3c/HTMLSourcingInbandTracks/commit/2af51c751e473a40f50775f2a14e1213f635d649
https://github.com/w3c/HTMLSourcingInbandTracks/commit/69c0208a4a3302cb666ab1455197ae4fdfb4c514
Wording changes to the text track cue mapping section of MPEG-2.