[WIP] Use tag to generate list of security related posts (en)

This automates the generation of the news post list on the /security page.
Posts will be included in the list when they are marked in the YAML front matter like this:

~~~
...
date: 2013-05-14 13:00:00 +0000
tags: security
lang: en
~~~

I will modify the other languages accordingly when this is agreed upon (I already tested with `de`).