Moving selectPlaylist to PlaylistSelector in playlist-selector.js

This PR moves selectPlaylist() from videojs-hls.js to its own class in playlist-selector.js.
By doing so videojs-hls.js gets smaller and less cluttered. More importantly having a separate playlist-selector.js allows third-party developers to swap in their own selectPlaylist() implementations.

This is the piece that I think we could keep as part of the tech, or "orchestrator" as I've been referring to it. Or possibly finding somewhere else to modularize this function so we can keep PlaylistSelector even more simple.