Merge pull request croservices#38 from Xliff/master

- Adds ability to associate arbitrary data with a Cro::Message

lib/Cro/Message.pm6

@@ -2,6 +2,12 @@
 # application is processing. It might be a message from a message queue, a
 # ZeroMQ message, a HTTP request, a HTTP response, etc.
 role Cro::Message {
+    # cw: For OOB attributes associated with the messages. Consider
+    #     that there is no way to know who the message is from,
+    #     or where it is supposed to go.
+    has %.attributes;
+
+
     # Provides trace output for use with CRO_TRACE=1 and  `cro trace ...`.
     # Should include a developer-friendly view of the message. The default is
     # just to show the message type