[Improvement] Support Vue.use to register a component (#401)

packages/button/index.vue

@@ -30,15 +30,11 @@
 </template>
 
 <script>
-import Loading from '../loading';
+import { create } from '../utils';
 
-export default {
+export default create({
   name: 'van-button',
 
-  components: {
-    Loading
-  },
-
   props: {
     block: Boolean,
     loading: Boolean,
@@ -66,5 +62,5 @@ export default {
       }
     }
   }
-};
+});
 </script>