It appears that not all Apple implementations follow the HLS guidelines.
While the DEFAULT=NO for an audio track should be optional and default
to NO, in practice native HLS players Safari and iOS devices treat the
missing DEFAULT as a MAYBE.
Fixes#1169
Now text-based WebVTT also uses the generic media pipeline. This
converts the WebVttTextOutputHandler to a WebVttMuxer to be more
consistent with the other muxer types.
This also allows choosing between single-segment text and multi-segment.
Before, we would generate both and use single-segment for DASH and
multi-segment for HLS; but now you can choose between either and either
are supported in both DASH and HLS.
Change-Id: I6f7edda09e01b5f40e819290d3fe6e88677018d9