Merging Playlistify and SpotCon

* Added a playlist import plugin interface (IPlaylistImporter) that
allows anyone to build their own method of importing songs.
* I've included an example plugin in
PlaylistImporter\SimpleTextPlaylistImporter.cs.
* IPlaylistImporter and the included SimpleTextPlaylistImporter example
render Playlistify obsolete so I will be retiring that project.
* Added the ability to attempt to find any duplicates in a given list of
songs. It uses fuzzy matching so it's pretty good at weeding out
duplicates in large playlists.