added support for bootstrap 'navbar-text' element

adds a new menu item option :navbar_text which if set causes menu item to be rendered as a text link instead of a url and styled with navbar-text class as per http://getbootstrap.com/components/#navbar-text
```ruby
SimpleNavigation::Configuration.run do |navigation|
  navigation.renderer = SimpleNavigationRenderers::Bootstrap3
  navigation.items do |primary|
    primary.item :sample_text, "Sample Text", navbar_text: true
  end
end
```
will produce...
```html
<ul class="nav navbar-nav">
  <li>
    <p class="navbar-text">Sample Text</p>
  </li>
</ul>
```