Support for btn input addons

Adds support for http://getbootstrap.com/components/#input-groups-buttons
The reason I used `content_tag` is that I branched this feature from master rather than `additional-components`, which includes a `button` helper (present in #15). Completely missed the `button_to` helper will update #15 to use that. 

```ruby
<%= f.text_field :name, suffix: button_to('Go', action: 'submit') %>
```
will actually work now :)