Dropdown add transfer prop

But not support Nested Usage

examples/components/test.vue

@@ -1,7 +1,17 @@
 <template>
-    <Poptip trigger="hover" title="提示标题" :transfer="true" placement="left" content="提示内容">
-        <Button>hover 激活</Button>
-    </Poptip>
+    <Dropdown transfer>
+        <a href="javascript:void(0)">
+            下拉菜单
+            <Icon type="arrow-down-b"></Icon>
+        </a>
+        <Dropdown-menu slot="list">
+            <Dropdown-item>驴打滚</Dropdown-item>
+            <Dropdown-item>炸酱面</Dropdown-item>
+            <Dropdown-item disabled>豆汁儿</Dropdown-item>
+            <Dropdown-item>冰糖葫芦</Dropdown-item>
+            <Dropdown-item divided>北京烤鸭</Dropdown-item>
+        </Dropdown-menu>
+    </Dropdown>
 </template>
 <script>
     export default {