[Helper] Automatically link to routes in site_url(), redirect() and anchor()

[eluser]Phil Sturgeon[/eluser]
Should this be a URL helper or an extension to the Config class which is where site_url() functionality is actually kept?

I don't much like the idea of extending such a core class just to achieve something so tiny, but it would mean the site_url acts the same over the whole site and not just via the helpers.