add site_url placeholder

Issue Summary

it appears nothing is happening with this module, but i find it very handy and have a request for a new placeholder token.

all the existing tokens assume you are logged into your site already. if you aren't (and i rarely am since i manage a lot of sites), you go to 'page not found'.

what would be helpful is a generic site_url token, http://www.sitename.com, that takes you to the home page.

then i can modify that to add '/user' (@site_url/user) and now i'm at my login page which is where i need to be anyway. as it is, i manually add this, but it'd be easier using a token that works out of the box for any newly created site.

thanks.