Add callback in aREF Query

[nichtich]

For instance to fetch additional RDF graphs if needed.

my $query = RDF::aREF::Query->new('dct_creator.foaf_name@');
my @names = $query->apply( {
    _id => 'http://example.org/',
    dct_creator => "http://example.org/alice"
}, fetcher => sub { ... } ); # will fetch http://example.org/alice