title: Change another module state from one module in Vuex
body: <p>I have two modules in my vuex store.</p>
  <pre><code>var store = new Vuex.Store({
      modules: {
          loading: loading 
          posts: posts
      }
  });
  </code></pre>
  <p>In the module <code>loading</code>, I have a property <code>saving</code> which can be set either <code>true</code> or <code>false</code> and also have a mutation function named <code>TOGGLE_SAVING</code> to set this property.</p>
  <p>In the module <code>posts</code>, before and after fetching posts, I want to change the property <code>saving</code>. I am doing it by calling <code>commit('TOGGLE_SAVING')</code> from one of the actions in the <code>posts</code> module.</p>
  <pre><code>var getPosts = function (context) {
      context.commit(TOGGLE_LOADING);
  };
  </code></pre>
  <p>When it tried to commit, I got following error in the console</p>
  <pre><code>[vuex] unknown local mutation type: TOGGLE_LOADING, global type: posts/TOGGLE_LOADING 
  </code></pre>
  <p>How can I mutate state in another module using <code>commit</code>?</p>