Allow use of HTTP proxies listening on port 80

Since port 80 is the default port for the HTTP protocol, `url.port()` returns `None`, causing an "invalid proxy" message.  Using `port_or_known_default()` will only return `None` if the both the port has been omitted and an unknown protocol has been specified.

src/main.rs

@@ -287,7 +287,7 @@ fn setup(args: &[String]) -> Setup {
                 |s| {
                     match Url::parse(&s) {
                         Ok(url) => {
-                            if url.host().is_none() || url.port().is_none() {
+                            if url.host().is_none() || url.port_or_known_default().is_none() {
                                 panic!("Invalid proxy url, only urls on the format \"http://host:port\" are allowed");
                             }