[Enhancement] Add song to playlist from Spotify menu

[3raxton]

Hey, @w0lfschild, thank you for all the hard work you've put into spotifyPlus and MacForge; they have incredibly enhanced my experience while using Spotify on my Mac.

I was thinking about enhancements and had the idea of adding the current song playing to a playlist through the two-finger click menu on Spotify's icon. I'm not sure how practical this is, but it could live under the second Spotify+ menu titled Add to Playlist, then on hover, it could display a list of playlists to add the song to.

I've seen this done in Javascript, but when it comes to Xcode, I have no idea how practical it is.

Whether implemented or not, I wanted to express my appreciation for the time put into spotifyPlus. Keep up the great work! 🎉