Add pods_image_default filter

Example use:
```
<?php
add_filter( 'pods_image_default', 'slug_pods_image_default_cb );
function slug_pods_image_default_cb( $default ) {
        //Use the image with the id of 179 as fallback, if no other default is set.
	$default = 179;
	return $default;
}
?>
```
With this filter added, presuming the image with the id of 129 doesn't exist, `pods_image( 129, $size = 'thumbnail' );` would return image id 179, while `pods_image( 129, $size = 'thumbnail', $default = 111 );` would return image 111.