Docs: add English language support (#170)

* feat: support lang entry build

* feat: vant support lang switch

* move lang iframe-router to utils & fix router link bug

* add en-US config && add some translation

* chang async. to async_ (support superman cdn)

* add layout translation

* change nav style

* upgrade zan-doc

* fix: doc config

* upgrade zan-doc && remove useless code

* fix changelog generate path

docs/examples-docs/zh-CN/loading.md

@@ -0,0 +1,44 @@
+<style>
+.demo-loading {
+  .van-loading {
+    display: inline-block;
+    margin: 10px 0 10px 20px;
+  }
+}
+</style>
+
+## Loading 加载
+
+### 使用指南
+``` javascript
+import { Loading } from 'vant';
+
+Vue.component(Loading.name, Loading);
+```
+
+### 代码演示
+
+#### 单色 spinner
+
+:::demo 单色 spinner
+```html
+<van-loading type="circle" color="black"></van-loading>
+<van-loading type="circle" color="white"></van-loading>
+```
+:::
+
+#### 渐变色 spinner
+
+:::demo 渐变色 spinner
+```html
+<van-loading type="gradient-circle" color="black"></van-loading>
+<van-loading type="gradient-circle" color="white"></van-loading>
+```
+:::
+
+### API
+
+| 参数       | 说明      | 类型       | 默认值       | 可选值       |
+|-----------|-----------|-----------|-------------|-------------|
+| color | 颜色 | `String` | `black` | `black` `white`   |
+| type | 类型 | `String` | `gradient-circle` | `gradient-circle` `circle`   |