Add initialRouteKey for StackRouter (#3540)

* use initialRouteName as key when initializing StackRouter

* fix null headerLeft

* merge back

* fixed tests

* use config flag

* fixed snapshots

* implemented requested changes

flow/react-navigation.js

@@ -362,6 +362,7 @@ declare module 'react-navigation' {
     initialRouteParams?: NavigationParams,
     paths?: NavigationPathsConfig,
     navigationOptions?: NavigationScreenConfig<*>,
+    initialRouteKey?: string,
   |};
 
   declare export type NavigationStackViewConfig = {|