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.

bandwidthVariance used to be a global var in videojs-hls.js