Link handling

Implementing the plugin as shown:

```
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
routes: {
"/": (_) => new WebviewScaffold(
url: "http://wckethman.com/htree/my-app/www/index.html",
withJavascript: true,
)
},
);
}
}
This works fine but when clicking on tel: links "net::ERR_UNKNOWN_URL_SCHEME" thrown
```

Any suggestions?

Start with the WebviewScaffold route shown in the issue and inspect how the plugin handles navigation for tel: links. Reproduce the net::ERR_UNKNOWN_URL_SCHEME error, then verify that tel: links are handled without that error and that the existing webview behavior remains intact.

Written by the indexing model from the issue text.