Add support for setting viewids in xml

[migelfn]

I think it would be nice have an ability to set view ids in xml

<com.objectlife.statelayout.StateLayout 
  android:id="@+id/sl_layout_state"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  app:content="@+id/v_content"
  app:error="@+id/v_error"
  app:empty="@+id/v_empty"
  app:loading="@+id/v_loading" 
  app:initialViewState="loading"
>

Also having default ids would be helpful too

Thanks!