Add RandomMask (not sure if really useful)

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;
+    }
+}