refactor($ga): using define

Author: ulivz

packages/@vuepress/plugin-google-analytics/index.js

@@ -1,18 +1,11 @@
 const path = require('path')
 
 module.exports = (options = {}, context) => ({
-  ready () {
+  define () {
     const { siteConfig = {}} = context
     const ga = options.ga || siteConfig.ga
-    context.GA_ID = ga ? JSON.stringify(ga) : false
-  },
-
-  chainWebpack (config) {
-    config.plugin('injections').tap(([options]) => [
-      Object.assign(options, {
-        GA_ID: context.GA_ID
-      })
-    ])
+    const GA_ID = ga || false
+    return { GA_ID }
   },
 
   enhanceAppFiles: [