Support singles (Songs not in an album)

[fineless71]

I know this is already a known issue but I didn't find any github issue about it. Not supporting singles is really the only major problem I have with LRCLIB as everything else can be solved by using LRCGET only for downloading and then using a different program for playback. But not supporting singles means I miss out on a lot of lyrics for songs and can't be fixed some other way.