Feature request: ability to hook `RCTRootView` and/or `RCTBridge` initialization.

[tom-un]

It would be useful for consumers to be able to hook into the creation of the RCTRootView and/or RCTBridge objects with custom code. For example, if the consumer wanted to run some custom code to build an initialProperties NSDictionary to pass to the RCTRootView or RCTBridge init. Or if some customization needed to be done to the RCTRootView such as changing properties on the view.