category-{slug}.php template

[scribu]

If a user needs to style a particular category differently, he can either use conditional code inside the category.php template, or make a category-{id}.php template.

The problem with the later is that when applying the theme to a different site, the category id will have likely changed.

That's why we need to use the category slug instead, so a theme can have something like this:

• category.php

• category-projects.php

• category-announcements.php

etc.

Reviewing this now.

(In [11814]) Support location of category templates based on category slug as well as id. Fixes #10614 based on patch from scribu.