title: Access URL in Redux reducer
body: <p>I have a language reducer that has an initial locale value:</p>
  
  <pre><code>const LanguageReducer = function(
    state = { locale: "en", messages: {} },
    action = {}
  ) {
    switch (action.type) {
      case CHANGE_LANGUAGE_SUCCESS:
      ...
        return state;
      default:
        return state;
    }
  };
  </code></pre>
  
  <p>The initial value of locale comes from the current URL. How can I access the URL and locale param in my reducer to initial the locale? I am using React-Router v4</p>