Extract "navigation" from Stack

[grabbou]

We should be able to easily share "navigation" prop between different navigators.

[czystyl]

But remember that not all the navigator have the same shape.

[grabbou]

Yes, I am thinking of the api:

StackNavigator.Navigation.(push|pop|whatever)(navigation)

where "navigation" is of ReactNavigation.Native.NavigationProp or something.

[czystyl]

Yep, and could be done with #22.