hls: adaptivedemux: Add support for EXT-X-START tag

Submitted by Hosang Lee

Link to original bug (#796434)

Description

Support EXT-X-START tag to start playing from a preferred point indicated in the playlist.

For EXT-X-START tag information refer to:
https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.5.2