Sets this element's source URL for reloading and polling
When an element is reloaded, Unpoly will make a request from the URL
that originally brought the element into the DOM. You may use [up-source] to
use another URL instead.
Example
Assume an application layout with an unread message counter.
You use [up-poll] to refresh the counter every 30 seconds.
By default this would make a request to the URL that originally brought the
counter element into the DOM. To save the server from rendering a lot of
unused HTML, you may poll from a different URL like so:
<div class="unread-count" up-poll up-source="/unread-count">
2 new messages
</div>
