feat($plugin-google-analytics): set anonymize ip to true (#1153)

Add the anonymize ip setting to true in analytics configuration in order to be compliant with GDPR.

Author: vittominacori

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

@@ -17,6 +17,7 @@ export default ({ router }) => {
     })(window, document, 'script', 'https://www.google-analytics.com/analytics.js', 'ga')
 
     ga('create', GA_ID, 'auto')
+    ga('set', 'anonymizeIp', true)
     ga('send', 'pageview')
 
     router.afterEach(function (to) {