Add support for opening a custom url via --url option

I use a few hosts aliases when developing locally to facilitate some url-driven aspects of our app, so being able to open http://localfoo:9966/some/weird/path.html would be SUPER handy.

Really small change, obviously. Specifying the option type as `url` means `nopt` will actually verify that it's a valid URL and will not parse it if it's not. Whaddya say, 2 PRs merged in one day? :)