new options for cobra commands

- ws cobra_subscribe has a new -q (quiet) option
- ws cobra_subscribe knows to and display msg stats (count and # of messages received per second)
- ws cobra_subscribe, cobra_to_statsd and cobra_to_sentry commands have a new option, --filter to restrict the events they want to receive

ws/ixcobra/IXCobraConnection.cpp

@@ -429,12 +429,18 @@ namespace ix
     }
 
     void CobraConnection::subscribe(const std::string& channel,
-                                     SubscriptionCallback cb)
+                                    const std::string& filter,
+                                    SubscriptionCallback cb)
     {
         // Create and send a subscribe pdu
         Json::Value body;
         body["channel"] = channel;
 
+        if (!filter.empty())
+        {
+            body["filter"] = filter;
+        }
+
         Json::Value pdu;
         pdu["action"] = "rtm/subscribe";
         pdu["body"] = body;