Include WordPress-Extra ruleset

[Dominik Schilling]

File: https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/blob/develop/WordPress-Extra/ruleset.xml

We should use the more stricter ruleset which is defined in WordPress-Extra. But not that strict like escaping all translations. Those should be excluded.

[Pascal Birchler]

Absolutely. I didn't add it the first time around because it usually generates a lot of false positives regarding unescaped output when adding to an existing project. You have to inspect each and every line to make sure it's safe.

Also, at least to some degree I want to include the WordPress-Docs ruleset as well. Our inline docs are often times really inconsistent, ranging from superb to non-existent.