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.

Switch to my own librespot-golang fork.

The librespot API does not allow advanced search therefore
we filter the results according to our advanced patterns.

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.