Allow user to get url for web source (#29)

When implementing a custom source that wraps web source, I'd like to
be able to retrieve the url of the web source. This is useful so I can
cleanly log which web source is returning an error when there are
multiple.

src/source.rs

@@ -25,6 +25,10 @@ impl WebSource {
     pub fn builder() -> WebSourceBuilder {
         WebSourceBuilder::new()
     }
+
+    pub fn url(&self) -> &Url {
+        &self.url
+    }
 }
 
 #[async_trait]