Add updatePlaylist method, docs and tests.

[sab666]

I'm developing a player that uses playlists which can be reordered. In order to pass these changes to Amplitude, a new playlist method is required. (In theory I could just create new playlists with a new hash as a name, but that feels hackish. More importantly: it would lose the state of the active_index, so navigation will become unreliable)

It reuses existing code to avoid duplication, and the tests and docs have been updated.