This makes it possible to specify metadata keys for the albums_sort_attr
and medias_sort_attr configuration options. This is done by prefixing
the key with 'meta.', so e.g. albums_sort_attr = 'meta.order' will sort
albums on the 'order' metadata key (collected from the .md files in each
directory).
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>