Universal Analytics plugin, with conditional inclusion in base.html

This plugin is designed to include [Google Universal Analytics](https://developers.google.com/analytics/devguides/collection/analyticsjs/) tracking code for use in templates as `{{ universalAnalytics }}`. There are two requirements I needed it to fulfill:

1. Put the tracking code _on the page_, not in an included .js file.
2. Not mess up template files with a bunch of ugly tracking code.

...which is why it performs a trivial replacement and includes the tracking code in the script itself. It does require the user to open the script and enter their tracking ID.