SYNOPSIS
# On the command line
catmandu convert Null --fix 'add_field(foo,bar)'
# creates { "foo": "bar" }
# In perl
use Catmandu::Importer::Null;
my $importer = Catmandu::Importer::Null->new();
my $n = $importer->each(sub {
my $hashref = $_[0];
# ...
});