Hotfix/33 generic action

[nifanic]

Fix for #33 generalizes Action type.

To specify custom action type:

1. Create new NewAction type with desired shape by extending provided Action type and passing in new NEW_TYPE_NAME type:

   interface NewAction extends Action<NEW_TYPE_NAME> {
     payload: any;
   }

2. Use reduce-reducers as before, but pass in NewAction, second type parameter:

   import reduceReducers from 'reduce-reducers';
   
   const initialState = { A: 0, B: 0 };
   
   const rootReducer = reduceReducers<NewState, NewAction>(
     initialState,
     addReducer,
     multReducer
   );

Your rootReducer is ready.