Enable clear button.

mastermind.html

@@ -94,7 +94,7 @@
           for (var i = 0; i < this.game.numLetters; i++) {
             this.locks[i].checked = false;
           }
-          clearUnlocked();
+          this.clearUnlocked();
         }
 
         clearUnlocked() {
@@ -148,6 +148,8 @@
           this.autoSubmit = document.getElementById('auto_submit');
           document.getElementById('submit')
                   .addEventListener('click', _ => ui.submitWord());
+          document.getElementById('clear')
+                  .addEventListener('click', _ => ui.clearAll());
 
           var entry = document.getElementById('letters_entry');
           clearElement(entry);