Adds Proxy::no_proxy(url) (#1694)

Adds the ability to add a NoProxy List to a Proxy via API, utilising the already existing functionality.
Makes NoProxy public and replaces NoProxy::new() with NoProxy::from_env() and NoProxy::from_string(). Adds a Proxy::no_proxy() method.

Closes #1690

src/lib.rs

@@ -295,7 +295,7 @@ if_hyper! {
     pub use self::async_impl::{
         Body, Client, ClientBuilder, Request, RequestBuilder, Response, Upgraded,
     };
-    pub use self::proxy::Proxy;
+    pub use self::proxy::{Proxy,NoProxy};
     #[cfg(feature = "__tls")]
     // Re-exports, to be removed in a future release
     pub use tls::{Certificate, Identity};