Add file association for playlists, BLC sets, favorites, karas...
With #533 (closed) fully implemented now, we can have file associations set up for us.
What this means is that we can tell the OS that KM handles some specific file extensions when you double click on these files in Windows Explorer/macOS Finder
.kmplaylist.json-> Could import the playlist in question
.kmfavorites.json-> Could import the favorites automatically
.kara.json-> Could play the song immediately upon starting up Karaoke Mugen without adding it to a playlist.
.kmblc.json-> Could import blacklist criteria sets (see #599)
This would allow people to share playlists and BLC sets easily. We could even put them on KM's website for download (not automatically, although this could come later)
Also, this is the 600th issue :)