fix(NavBar): don't render custom BackButton if root nav state (#998)
fix(NavBar): don't render custom BackButton if root nav state (#998)

Avoid rendering a custom `BackButton` if the default implementation
should avoid rendering, such as when the current scene is the root
scene. Since main `state` isn't being passed to the custom component
we have no way (that I'm aware of) of determining whether it should
actually render a back button.
peteychuk/react-native-router-flux,Wondermall/react-native-router-flux,gectorat/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,peteychuk/react-native-router-flux,charpeni/react-native-router-flux,dmazin/react-native-router-flux,charpeni/react-native-router-flux,gijoehosaphat/react-native-router-flux,dmazin/react-native-router-flux,mcabs3/react-native-router-flux,gectorat/react-native-router-flux,mcabs3/react-native-router-flux,Wondermall/react-native-router-flux,gectorat/react-native-router-flux,gijoehosaphat/react-native-router-flux,aksonov/react-native-router-flux,dmazin/react-native-router-flux,Wondermall/react-native-router-flux,peteychuk/react-native-router-flux,gijoehosaphat/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,mcabs3/react-native-router-flux,charpeni/react-native-router-flux,Swordsman-Inaction/react-native-router-flux