for flexible format support

```
# disable format, default = enable
JSV::Validator->new(enable_format => 0);
```

```
# replace formats, entirely
JSV::Validator->new(
    formats => +{
        original_format => sub {
            my $instance = shift;
            return $instance eq "omoro-";
        }
    }
);
```

```
# add new format or override existing format
$jsv_instance->register_format("original_format" => sub {
    my $instance = shift;
});
```

みたいな感じで。