Returns the URL the given element was retrieved from.
If the given element was never directly updated, but part of a larger fragment update, the closest known source of an ancestor element is returned.
In the HTML below, the element
#one was loaded from the URL
<div id="one" up-source"/foo"> <div id="two">...</div> </div>
We can now ask for the source of an element:
up.fragment.source('#two') // returns '/foo'
The element or CSS selector for which to look up the source URL.