Add example "cureq". (#330)

Contrary to smoke-test, this takes full URLs on the commandline and
prints their contents to stdout. This makes it easier to test behavior
with specific URLs. I hope to later add flags for various behaviors like
printing headers, following redirects, enabling / disabling cookies,
and verbose output.

Also add a useful debug line when receiving a cookie header.

src/unit.rs

@@ -411,6 +411,10 @@ fn save_cookies(unit: &Unit, resp: &Response) {
         return;
     }
     let cookies = headers.into_iter().flat_map(|header_value| {
+        debug!(
+            "received 'set-cookie: {}' from {} {}",
+            header_value, unit.method, unit.url
+        );
         match Cookie::parse(header_value.to_string()) {
             Err(_) => None,
             Ok(c) => Some(c),