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/lib.rs

@@ -55,6 +55,8 @@ pub use crate::client::Client;
 pub use crate::error::{Error, Result};
 pub use crate::server::Server;
 
+#[macro_use]
+mod cfg;
 #[macro_use]
 mod common;
 pub mod body;