Add RandomMask (not sure if really useful)

src/main/java/com/sk89q/worldedit/WorldEdit.java

@@ -632,6 +632,10 @@ public class WorldEdit {
             }
             return new BiomeTypeMask(biomes);
 
+        case '%':
+            int i = Integer.parseInt(component.substring(1));
+            return new RandomMask(((double) i) / 100);
+
         case '!':
             if (component.length() > 1) {
                 return new InvertedMask(getBlockMaskComponent(player, session, masks, component.substring(1)));