Links that are currently loading through Unpoly
are assigned the .up-active class automatically.
Style .up-active in your CSS to improve the perceived responsiveness
of your user interface.
The .up-active class will be removed when the link is done loading.
Example
We have a link:
<a href="/foo" up-follow>Foo</a>
The user clicks on the link. While the request is loading,
the link has the up-active class:
<a href="/foo" up-follow class="up-active">Foo</a>
Once the link destination has loaded and rendered, the .up-active class
is removed and the .up-current class is added:
<a href="/foo" up-follow class="up-current">Foo</a>
