feat(http2): Make HTTP/2 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/2 methods and support behind an
  `http2` cargo feature, which will not be enabled by default. To use
  HTTP/2, add `features = ["http2"]` to the hyper dependency in your
  `Cargo.toml`.

src/cfg.rs

@@ -0,0 +1,9 @@
+macro_rules! cfg_http2 {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "http2")]
+            //#[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
+            $item
+        )*
+    }
+}