Add variables for default metadata and image license

src/MDLicensesSet.php

@@ -11,6 +11,9 @@ declare(strict_types = 1);
  */
 final class MDLicensesSet extends MDValueSet {
 
+    const DEFAULT_LICENSE_IMGS = 'CC BY-NC-SA';
+    const DEFAULT_LICENSE_METADATA = 'CC BY-NC-SA';
+
     const AVAILABLE_LICENSES = [
 
         'CC BY-NC-SA'           => 'https://creativecommons.org/licenses/by-nc-sa/4.0/',