[add] NativeModules shim

The `NativeModules` namespace is import by some libraries, which will
throw an error if missing.

Close #86

src/index.js

@@ -29,6 +29,9 @@ import TextInput from './components/TextInput'
 import Touchable from './components/Touchable'
 import View from './components/View'
 
+// modules
+import NativeModules from './modules/NativeModules'
+
 const ReactNative = {
   // apis
   Animated,
@@ -58,6 +61,9 @@ const ReactNative = {
   TouchableWithoutFeedback: Touchable,
   View,
 
+  // modules
+  NativeModules,
+
   // React
   ...React,
   ...ReactDOM,

src/modules/NativeModules/index.js

@@ -0,0 +1,3 @@
+// NativeModules shim
+const NativeModules = {}
+export default NativeModules