[addon attach] Fix reference error

Also bump version to 0.21

addons/attach/attach.js

@@ -33,10 +33,10 @@
         socket.send(data);
       };
 
-      socket.addEventListener('message', _getMessage);
+      socket.addEventListener('message', term._getMessage);
 
       if (bidirectional) {
-        this.on('data', _sendData);
+        this.on('data', term._sendData);
       }
       
       socket.addEventListener('close', term.detach.bind(term, socket));