Update features to reflect BAPS3 spec.

See [Player][] in the BAPS3 spec; this also brings the features in line with those mentioned in the [command reference][].

In before @LordAro complains about `FEATURES` :grinning:

[Command reference]: https://github.com/UniversityRadioYork/ury-playd/blob/851187d2348bb2f33d4437a484768451e9250099/README.commands.md
[Player]: http://universityradioyork.github.io/baps3-spec/services/player.html