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/src/utils/lang.js

@@ -0,0 +1,16 @@
+const userLang = window.localStorage.getItem('VANT_LANGUAGE') || window.navigator.language || 'en-US';
+let defaultLang = 'en-US';
+if (userLang.indexOf('zh-') !== -1) {
+  defaultLang = 'zh-CN';
+}
+
+let currentLang = defaultLang;
+
+export function getLang() {
+  return currentLang;
+}
+
+export function setLang(lang) {
+  window.localStorage.setItem('VANT_LANGUAGE', lang);
+  currentLang = lang;
+}