Added /clearhistory and /clearclipboard.

src/WorldEditSession.java

@@ -34,6 +34,14 @@ public class WorldEditSession {
     private int historyPointer = 0;
     private RegionClipboard clipboard;
 
+    /**
+     * Clear history.
+     */
+    public void clearHistory() {
+        history.clear();
+        historyPointer = 0;
+    }
+
     /**
      * Get the edit session.
      *