add google optimize container (#23)

Author: shoota

themes/apollo/layout/_partial/head.ejs

@@ -34,6 +34,7 @@
         })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 
         ga('create', 'UA-45958410-1', 'auto');
+        ga('require', 'GTM-KX3B57C');
         ga('require', 'linker');
         ga('linker:autoLink', ['github.com'], false, true);
         console.log('use linker plugin');