Added active selector support for tags.

* Allows tags to be marked as "active" when search and target values
  match.
* Usage: a "Example", {href: "/example"}, {search_value: "/example"}.
* Yields: <a href="/example" class="active"></a>
Added a tag activator object for detecting which tags to activate.
Refactored Tag to use TagActivator instead of hash settings.