Fix html-jsx compiler

It changed React Playground to add a required props but unfortunately didn't update the call sites of the front-page. I don't think it should be required so I'm just making it optional and providing the correct default value.

Test Plan:
 - Open the front page and make sure examples are working
 - Open /react/jsx-compiler.html and make sure it is working
 - Open /react/html-jsx.html and make sure it is working

docs/_js/live_editor.js

@@ -57,10 +57,18 @@ var ReactPlayground = React.createClass({
 
   propTypes: {
     codeText: React.PropTypes.string.isRequired,
-    transformer: React.PropTypes.func.isRequired,
+    transformer: React.PropTypes.func,
     renderCode: React.PropTypes.bool,
   },
 
+  getDefaultProps: function() {
+    return {
+      transformer: function(code) {
+        return JSXTransformer.transform(code).code;
+      }
+    };
+  },
+
   getInitialState: function() {
     return {mode: this.MODES.XJS, code: this.props.codeText};
   },