Spotify Player support arrives in v2.1.6

[maboa]

A beta version of the Spotify player is now available, unfortunately the Spotify iFrame API is somewhat limited and so we don't quite have the full functionality of other players.

The API limits us to:

1. A seek accuracy of nearest second
2. An update frequency of one second (although a workaround is provided)
3. Playing a file without previous interaction will always play from start
   ie – a shared selection will highlight but not start at the start of that selection.

Here's an example of it in action https://hyperaudio.github.io/hyperaudio-lite/spotify.html Keen to hear what people think, especially @andreashorn who filed the issue requesting it. :)