This is the main component required to enable WebVTT subtitles in HLS.
It passes through each WebVTT file as a sample, and derives the correct,
adjusted timestamp for each of them on the way through.
Not yet wired up because we need to properly share the same
PtsTimestampAdjuster everywhere, and also stop instantiating new instances
of the adjuster. The adjuster will also need to correctly handle
discontinuities, since we'll no longer be creating new instances of it.
Issue: #151
c2df814b58