User#playlist support for limit, offset and private playlists

I apologize for having these two fixes in one pull request but they are related to the same part of code.

First commit adds [support for limit and offset](https://developer.spotify.com/web-api/get-list-users-playlists/#tablepress-132) for playlists.

Second commit adds checking if the user is authorized using OAuth so it sends proper headers and returns list of playlists including private ones.