hls: make mpegurl type available to api (#197)

- simplifies type completion
- mpegurl avoids the need to quote the type property name:
  play([{mpegurl: "example.m3u8"}])
- type completion in dedicated function and put to use in video tag
  cache
- default name application/x-mpegurl avoids potential proprietary
  constraints with application/vnd.apple.mpegurl (which is still
  recognized)