title: Show CircularProgressIndicator while loading URL in Web View
body: <p>I want to show <code>CircularProgressIndicator</code> when ever the <code>webview</code> loads an URL. Below is code but it only shows loading element while initializing the <code>webview</code>.</p>
  
  <pre><code>Widget build(BuildContext context) {
  
      return new MaterialApp(
        theme
          : new ThemeData(
          primaryColor: Color.fromRGBO(58, 66, 86, 1.0), fontFamily: 'Raleway'),
        routes: {
          "/": (_) =&gt; new WebviewScaffold(
            url: url,
            appBar: new AppBar(
              title: Text(title),
              actions: &lt;Widget&gt;[
                IconButton(
                  icon: Icon(Icons.close),
                  onPressed: () =&gt; Navigator.of(context).pop(null),
                )
              ],
            ),
            withJavascript: true,
            withLocalStorage: true,
            appCacheEnabled: true,
            hidden: true,
          )
        },
      );
    }
  </code></pre>
  
  <p>I want it to show loading element when user clicks on any link within <code>webview</code>.</p>