feat(plugins): add Google Analytics plugin (#66)

docs/README.md

@@ -468,7 +468,7 @@ If a document can have a search, can enhance some user experience. Installing th
 
 
 ```html
-<script src="//unpkg.com/docsify/lib/docsify.js"></script>
+<script src="//unpkg.com/docsify"></script>
 <script src="//unpkg.com/docsify/lib/plugins/search.js"></script>
 ```
 
@@ -495,3 +495,20 @@ window.$docsify = {
   }
 }
 ```
+
+### Google Analytics
+
+Install the plugin and configure the track id.
+
+```html
+<script src="//unpkg.com/docsify" data-ga="UA-XXXXX-Y"></script>
+<script src="//unpkg.com/docsify/lib/plugins/ga.js"></script>
+```
+
+or
+
+```js
+window.$docsify = {
+  ga: 'UA-XXXXX-Y'
+}
+```

docs/zh-cn.md

@@ -501,3 +501,20 @@ window.$docsify = {
   }
 }
 ```
+
+### Google Analytics
+
+安装插件并且配置 track id。
+
+```html
+<script src="//unpkg.com/docsify" data-ga="UA-XXXXX-Y"></script>
+<script src="//unpkg.com/docsify/lib/plugins/ga.js"></script>
+```
+
+或者
+
+```js
+window.$docsify = {
+  ga: 'UA-XXXXX-Y'
+}
+```