Add support for WebVTT in HLS
So far, we don't have support for WebVTT in HLS because we cannot handle empty segments/cue and absolute time yet. This works for DASH though.
Sample URL: https://live.performa.intio.tv/media/635c191e-b253-464e-911c-5510b2a80a74/master-allsubs.m3u8