另类从 view 内 push 的方法

摘自http://blog.sunnyxx.com/2015/12/19/self-manager-pattern-in-ios/
iOS 开发中的 Self-Manager 模式

// 假设 App 结构是 Root -> TabBar -> Navigation -> ViewController
UITabBarController *tabBarControler = (id)UIApplication.sharedApplication.delegate.window.rootViewController;
UINavigationController *navigationController = tabBarControler.selectedViewController;