Fixed bug when tabs are not siblings.

Hello, 

My HTML structure looks like this : 

```html
<!-- Toggles -->
<ul>
<li>
  <a data-tab="#tab1">Toggle 1</a>
</li>
<li>
  <a data-tab="#tab2">Toggle 2</a>
</li>
</ul>
<!-- Tabs -->
<div class="someClass">
  <div id="tab1">
    Tab content
  </div>
</div>
<div class="someClass">
  <div id="tab2">
    Tab content
  </div>
</div>
```

Tabby is not able to toggle the tabs correctly because it targets the element's siblings. 

I changed the `hideOtherTabs` function to rely only on data attributes.