Pass limit and offset parameters in GetPlaylists method

[lucasfogliarini]

GetPlaylists is not passing limit and offset parameters to GetPlaylists:
https://github.com/Ringobot/SpotifyApi.NetCore/blob/master/src/SpotifyApi.NetCore/PlaylistsApi.cs#L54

Can you fix it for us?

[Natfan]

I have raised #57 in an attempt to resolve this issue.