Add dispose terminal button to demo

demo/index.html

@@ -32,6 +32,7 @@
     </div>
     <hr/>
     <p><strong>Attention:</strong> The demo is a barebones implementation and is designed for the development and evaluation of xterm.js only. Exposing the demo to the public as is would introduce security risks for the host.</p>
+    <button id="dispose" title="This is used to testing memory leaks">Dispose terminal</button>
     <script src="dist/bundle.js" defer ></script>
   </body>
 </html>

demo/main.js

@@ -76,6 +76,12 @@ function createTerminal() {
   term.fit();
   term.focus();
 
+  document.getElementById('dispose').addEventListener('click', () => {
+    term.dispose();
+    term = null;
+    window.term = null;
+  });
+
   // fit is called within a setTimeout, cols and rows need this.
   setTimeout(function () {
     initOptions(term);