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/en-US/quickstart.md

@@ -0,0 +1,49 @@
+## Vant
+
+Mobile UI Component based on `Vue 2.0`
+
+### Install
+
+```shell
+npm i vant -S
+```
+
+### Usage
+
+#### 1. Use [babel-plugin-import](https://github.com/ant-design/babel-plugin-import) (推荐)
+```bash
+# Install babel-plugin-import
+npm i babel-plugin-import -D
+```
+
+```js
+// set babel config in .babelrc or babel-loader
+{
+  "plugins": [
+    ["import", { "libraryName": "vant", "style": true }]
+  ]
+}
+```
+
+Then you can import components from vant, equivalent to import manually below.
+
+```js
+import { Button } from 'vant';
+```
+
+#### 2. Manually import
+
+```js
+import { Button } from 'vant/lib/button';
+import 'vant/lib/vant-css/button.css';
+```
+ 
+#### 3. Import all components
+
+```js
+import Vue from 'vue';
+import Vant from 'vant';
+import 'vant/lib/vant-css/index.css';
+
+Vue.use(Vant);
+```