Add song and playlist types. Check `vuiet--ivy-play-song' arg type.

[firmart]

Fix #53.

• Define song type (e.g. ("Artist" "title")) and playlist type (e.g. '(song1 song2 ...))
• Define vuiet--check-playlist which type-check its argument and try to fix a song to a playlist.

The one-liner (setq songs (vuiet--check-playlist songs)) can be added to other functions which may have the same issue.

[mihaiolteanu]

53 fixed with a simpler solution.