allow specifying a url name as the "from path" on a redirect

This allows you to pre-define a named url using `url` and then use
that url in a later redirect.  The net-effect is you can name urls
used in redirects like you would name urls used in routing.
Merge pull request #91 from redding/kr-addurl-redir

allow specifying a url name as the "from path" on a redirect