make HeaderName and HeaderValue publicly accessible (#436)

It's possible that users have a different version of hyper included, so export
HeaderName and HeaderValue to allow them to populate client::Config.

It's also possible that they don't have it imported, so this would be convenient
for them.

wstunnel/src/config.rs

@@ -1,5 +1,5 @@
 use crate::tunnel::LocalProtocol;
-use hyper::http::{HeaderName, HeaderValue};
+pub use hyper::http::{HeaderName, HeaderValue};
 use std::net::SocketAddr;
 use std::path::PathBuf;
 use std::time::Duration;