Add filter for metabox HTML output, similar to core's admin_post_thum…

…bnail_html filter.

multi-post-thumbnails.php

@@ -410,7 +410,7 @@ private function post_thumbnail_html($thumbnail_id = null) {
 				$content .= sprintf('<script>%s</script>', $modal_js);
 			}
 
-			return $content;
+			return apply_filters( 'mpt_admin_post_thumbnail_html', $content, $post_ID, $thumbnail_id, $this->post_type, $this->id );
 		}
 
 		/**