v2.1.0

• Modify Discarded function to be explicit on what to do in case of discarding the message #66

This is backwards incompatible change but it just requires this change:

- config.Discarded = func(msg *sarama.ConsumerMessage, err error) {
-                       ... discarded stuff
-                    }
+ config.Discarded = func(ctx context.Context, msg *sarama.ConsumerMessage, err error) bool {
+                       ... discarded stuff
+                       return true
+                    }