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)));

src/main/java/com/sk89q/worldedit/masks/RandomMask.java

@@ -0,0 +1,24 @@
+package com.sk89q.worldedit.masks;
+
+import com.sk89q.worldedit.EditSession;
+import com.sk89q.worldedit.LocalPlayer;
+import com.sk89q.worldedit.LocalSession;
+import com.sk89q.worldedit.Vector;
+
+public class RandomMask implements Mask {
+
+    private final double ratio;
+
+    public RandomMask(double ratio) {
+        this.ratio = ratio;
+    }
+
+    @Override
+    public void prepare(LocalSession session, LocalPlayer player, Vector target) {
+    }
+
+    @Override
+    public boolean matches(EditSession editSession, Vector pos) {
+        return Math.random() < ratio;
+    }
+}