Playlist system

[icedream]

Implement a way to append songs and go forward/backwards. This way we can pretty much just queue songs and let the bot handle the sequential playback instead of having to manually type "play" all the time.

• Implement "append" ("enqueue"/"add") command
• Implement "next" command
• Implement "previous" command
• Implement separate commands to clear the playlist
• Implement support for URLs linking to playlists

[icedream]

Before I gonna tackle this I will do issue #17 so we get more grip on the playlist data.