add Selector functionality

lib/potent-reducer.js

@@ -26,7 +26,8 @@ export const createStore = options => {
     return React.createElement(context.Provider, { value: store }, children)
   }
   const useStore = () => useContext(context)
-  return { Provider, Consumer: context.Consumer, useStore }
+  const useSelector = selector => selector(useContext(context)[0])
+  return { Provider, Consumer: context.Consumer, useStore, useSelector }
 }
 
 /** Turn a reducer definition object to a function