Add Play Next Song and Play Previous Song functionality

[guidokessels]

• Use cmd-shift-right to play the next song
• Use cmd-shift-left to play the previous song
• Both commands can also be accessed via Packages -> Spotify in the menu
• Fix an error when spotify.getTrack couldn't retrieve the current track

[jakemarsh]

Wow, really cool! Thanks so much @guidokessels! :smile: :+1:

[guidokessels]

Awesome! :metal: