Add support for custom ACF fields

We are currently using this
https://github.com/joppuyo/acf-image-aspect-ratio-crop

which creates a field type `image_aspect_ratio_crop`

I can seemingly use it by adapting `FieldFactory.php`

```
case 'image':
case 'img':
case 'image_aspect_ratio_crop': // <= added this
$field = new Image($post);
```

Would be good to be able to define that dynamically, but I'm guessing the internal field mapping won't necessarily be the same?