Add Flow libdef (react-navigation#3391)

* Add Flow libdef

* Move libdef to flow folder in root

* Update libdef with some refinements discovered while writing test

Mostly refining navigator config types. In the past when we tried it we got in-repo Flow errors. With a libdef we can now assert it as true!

* Get rid of all imports in libdef and refine some types

Author: Ashoat

.eslintignore

@@ -1,4 +1,5 @@
 coverage
+flow
 node_modules
 lib*
 build

examples/NavigationPlayground/.flowconfig

@@ -51,6 +51,7 @@
 node_modules/react-native/Libraries/react-native/react-native-interface.js
 node_modules/react-native/flow/
 node_modules/react-native/flow-github/
+../../flow/
 
 [options]
 module.system=haste