支持命名路由

[micross]

能否添加命名路由的支持，第二个参数作为路由的名称，类似这样：

Router::new()
    .path("writers")
    .get(list_writers, "list_writers")
    .post(create_writer, "create_writer")
    .push(
        Router::with_path("<id>")
            .get(show_writer, "show_writer")
            .patch(edit_writer, "edit_writer")
            .delete(delete_writer, "delete_writer")
            .push(Router::with_path("articles").get(list_writer_articles)),
    );

然后再添加个函数route，通过名称获取路径，类似：

Router::route("list_writers") 返回 /writers Router::route("create_writer") 返回 /writers Router::route("show_writer", 1) 返回 /writers/1