allow extra classes in className js function

web/app/themes/badegg/resources/js/blocks/lib/classNames.js

@@ -1,4 +1,4 @@
-export function containerClassNames(attributes, bgProps)
+export function containerClassNames(attributes, extraClasses = [])
 {
 
   let classNames = [
@@ -11,6 +11,9 @@ export function containerClassNames(attributes, bgProps)
   if('alignment' in attributes)
     classNames.push(`align-${attributes.alignment}`);
 
+  // combine arrays
+  classNames = classNames.concat(extraClasses);
+
   return classNames;
 }