Added Soundcloud Downloader plugin

Added a plugin that enables Soundcloud downloads (based on [my SCDL project](https://github.com/joengelm/scdl)).
    
    scdl https://soundcloud.com/ohwondermusic/all-we-do
    download https://soundcloud.com/ohwondermusic/all-we-do from Soundcloud
    
These downloads go to "~/Music/Soundcloud/".