When Unpoly inserts a fragment, the [up-source] attribute is automatically set
to the URL from which the fragment's HTML was loaded.
When an element is reloaded, Unpoly will request the URL from the closest
[up-source] attribute. You may manually set [up-source] attribute to indicate a different
source URL for a fragment or a fragment's descendant.
To access the source URL from JavaScript, use up.fragment.source().
