Feature: append HTML classes to a category section via html_class option

I'd like to qualify tables in `<section>` tags of categories, so I added a new method to do this.

Usage:

```rb
category do
  id "Local Variables"
  html_class "simple-listing"
end
```

Result:

```html
<section class="category simple-listing">
<!-- ... -->
</section>
```

---

I added this feature for my own cheat sheet, so that I can apply different styles on some categories: chitsaou/font-awesome-cheatsheet@793b3808e8cb5e27c790d2c8c5b1fd66ae89a718