Using standard Icons for NavigationBar in Xamarin Forms

Hi!

Is there a way to use OS specific icons on a certain platform in Xamarin Forms?

For example, on iOS I would like to use UIBarButtonSystemItemAdd and on WP/droid a custom image.

  <ContentPage.ToolbarItems>
    <ToolbarItem Name="Add" Command="{Binding ShowAddSheet}" Icon=" magic here " />
  </ContentPage.ToolbarItems>

Thanks

// Johan