Use data-target attribute if present (Collapse extension)

This patch fixes a JS error when the element with the attribute data-toggle="collapse" has a URL href (instead of a jQuery selector) and additionally a data-target attribute with the jQuery selector for the collapsible element. This is useful if you want to offer the opportunity to open the link in a new window (or even when JS is disabled).
Example: 
<a data-toggle="collapse" href="http://www.example.org/" data-target="#collapsibleElement>
  Collapsible Group Item #1
</a>