adds option to add tracks to a playlist by string URIs, not just Track objects

[scottopell]

I have a use case where I store spotify URIs in my database and then would like to add them en-masse into a playlist.
So rather than create Track objects for each one, I modified Playlist#add_tracks! to take the first parameter as an array of Tracks or strings (and assumes if its a string then its a valid spotify URI)

Is this a change that you'd be interested in?

[guilhermesad]

Yep looks good, thanks for this!