Support for btn input addons

Adds support for http://getbootstrap.com/components/#input-groups-buttons
Since the objects that can be used as prefixes or suffixes are limited, I wouldn't want to expose the generic `content_tag` as the way to use prefixes or suffixes that are not plain text.

For instance, right now you can write 

```rhtml
<%= f.text_field :name, prefix: 'ok' %>
```

The next nice step would be 

```rhtml
<%= f.text_field :name, prefix: icon(:ok) $>
```

which I expect not to be working now, not being HTML-safe (I might be wrong, I didn't check).

So for the input group button, I'd like the syntax to be

```rhtml
<%= f.text_field :name, suffix: button_to('Go', action: 'submit') %>
```

which would be easier to write (and less prone to errors) than the full `content_tag`. What do you think?