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
How about adding a link to the description of Google Analytics is and why one would use it? Something like:

```
#-------------------------------
# Google Analytics Section
#
# Analytics help you understand who visits your website.
# Learn more at: http://www.google.com/analytics/why/

# NOTE: If you want to use Google Analytics…
```

It hopefully will save users a Google search. :thumbsup: