message: give access to Subscriptions crossbeam channel
diff: @@ -50,6 +50,34 @@ impl Subscription {
  }))
  }
  + /// Get a crossbeam Receiver for subscription messages.
  + /// Useful for crossbeam_channel::select macro
  + ///
  + /// # Example
  + /// ```
  + /// # fn main() -> std::io::Result<()> {
  + /// # let nc = nats::connect("demo.nats.io")?;
  + /// # let sub1 = nc.subscribe("foo")?;
  + /// # let sub2 = nc.subscribe("bar")?;
  + /// # nc.publish("foo", "hello")?;
  + /// let sub1_ch = sub1.receiver();
  + /// let sub2_ch = sub2.receiver();
  + /// crossbeam_channel::select! {
  + /// recv(sub1_ch) -> msg {
  + /// println!("Got message from sub1: {:?}", msg);
  + /// Ok(())
  + /// }
  + /// recv(sub2_ch) -> msg {
  + /// println!("Got message from sub2: {:?}", msg);
  + /// Ok(())
  + /// }
  + /// }
  + /// # }
  + /// ```
  + pub fn receiver(&self) -> &channel::Receiver<Message> {
  + &self.0.messages
  + }
  +
  /// Get the next message, or None if the subscription
  /// has been unsubscribed or the connection closed.
  ///
  