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.
Example
In the HTML below, the element #one was loaded from the URL /foo:
<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'
