Add in public openRoute method for Router

[jonmorehouse]

Resolves #11

Allows you to have shared route opening methods from the parent class containing the react-native-router instance.

[joeferraro]

thanks for this @jonmorehouse, needed this for an app with a global sidenav

[MossP]

Thanks for this @jonmorehouse . But how do you then call this from the parent?

edit Actually, I'm not sure that this is what I need. Does this allow you to change the internal route of the router from the parent class that called the --Router firstRoute={myRoute}-- ? I'd like to be able to popToRoot of the navigator when the TabBarButton is reselected. I have it working from children of the Router, but not from the parent.