add useDispatch hook

lib/potent-reducer.js

@@ -21,13 +21,15 @@ export const usePotentReducer = options => {
  */
 export const createStore = options => {
   const context = createContext(null)
+  const { Consumer } = context
   const Provider = ({ children }) => {
     const store = usePotentReducer(options)
     return React.createElement(context.Provider, { value: store }, children)
   }
   const useStore = () => useContext(context)
   const useSelector = selector => selector(useContext(context)[0])
-  return { Provider, Consumer: context.Consumer, useStore, useSelector }
+  const useDispatch = () => useContext(context)[1]
+  return { Provider, Consumer, useStore, useSelector, useDispatch }
 }
 
 /** Turn a reducer definition object to a function