Re-add #![feature(extern_crate_item_prelude)] to fix docs.rs build

Author: tylerreisinger

src/lib.rs

@@ -166,7 +166,8 @@
 //!         })
 //! }
 //! ```
-
+//!
+#![feature(extern_crate_item_prelude)]
 #![feature(proc_macro_diagnostic)]
 #![recursion_limit="128"]
 extern crate proc_macro;