provide option for maintaining horizontal postion

Derives position.left from immediate parent.
```
<div class="large-4 columns">
  <ul class="hide-for-small" data-magellan-expedition="contained">
    <li><a href="#">An Item</a></li>
  </ul>
</div>
```

Derives position.left from parent with selector "section#chapter-34"
```
<section id="chapter-34">
  <div class="large-4 columns">
    <ul class="hide-for-small" data-magellan-expedition="contained" data-magellan-reference="section#chapter-34">
      <li><a href="#">An Item</a></li>
    </ul>
  </div>
</section>
```