Support for btn input addons

Adds support for http://getbootstrap.com/components/#input-groups-buttons
Finally I got time to look at this.
I cannot think at a much better solution than what you suggest.

The Bootstrap documentation says:
> Buttons in input groups are a bit different and require one extra level of nesting. 
> Instead of .input-group-addon, you'll need to use .input-group-btn to wrap the buttons.

My understanding is that `.input-group-btn` should be used **only** if the content is **exactly** a button.
So one solution would be to scan the string for a regex like `%{^<button.+>.+</button>$}`.
However, as you point out, the content might be a different DOM element _looking like a button_, for instance

```html
<input type="submit" class="btn btn-default">
```

In any case, all I can think of is a regex-matching of `addon`, similar to what you suggest…