Support HLS streams

[danielvijge]

MixCloud has HLS streams available. When the PlayHLS plugin is installed, offer an option to play HLS instead of the lower quality HTTP streams.

• Add option to select preferred stream format (HLS-192, HLS-64, or HTTP)
• Retrieve HLS playlist using custom user agent in request (MixCloud CDN blocks the default iTunes user agent)
• Figure out who to use _fetchTrackExtra function to parse both HTTP streams, and HLS streams