Playlist can now be added to the initial play queue with the
-playlists command line flag or the playlists yaml key in the
config file.
To load a spotify playlist the link obtained when sharing the playlist
from spotify is used.

Storing song objects in the Songs slice and using pointers to the
underlying array slots breaks if the underlying array must be grown.
When the array is grown the objects are copied to the new array and
the pointers used in queue point to the old objects.
This causes that Playlist.applyTimeBonus may not correctly apply
to the most recent list of songs.

This is fixed by always using Song pointers.

Due to an logic error was _next() a second time from runMpv after
the mpv subprocess was terminated by the first Next() call.

Only the player.serveCmds and the runMpv goroutines are racing now.
But this race should be a benign race.