Added the default Google Analytics script.

Hey. I added the default Google Analytics script and the functionality to edit the UA-Code in settings.yml. It's included right before the closing </head> tag like recommended here: https://support.google.com/analytics/answer/1008080
Hey great idea! I would rather add this link: https://support.google.com/analytics/answer/1008065
because it is a introduction to Analytics and more valuable for beginners. :smiley: