Category template tag

Change the category tag so that you can do something like this:

```django
{% post_category pre_text="Categories: " outer_tag="p" outer_class="outer-class" inner_tag="ul" inner_class="inner-class" link_class="link-class" %}
```

Which will produce:

```html
<p class="outer-class">Categories: <ul class="inner-class"><li><a class="link-class" href="/category-1">Category 1</a></li></ul></p>
```