Adding random albums does not take into account artists. #159

When adding a number of random albums, only the 'album' tag is taken into account, resulting into adding several albums with the same name by different artists as a unique album. For example, if artists 'foo' and 'bar' and 'baz' have all made an album called 'Greatest Hits' or something, then ncmpcpp will add all three albums end to end as only one album. This behavior can also be seen when toggling the album separator (the action bound to '!'): two consecutive albums with the same name by different artists will not have a separator.