Sensu codec for RabbitMQ output

Originally reported by Brian Preston at https://logstash.jira.com/browse/LOGSTASH-2116

> I'm trying to get matching ingested logs to fire a sensu alert. One way I see to do this is have logstash output matching logs into rabbitmq, using the output already written. However the message would need to be formatted into the format sensu uses, which implies a codec.
>
> History: The google group "logstash-users" mentioned a need for this in Nov 2013, but it looks like no one has implemented it yet.

The original thread: https://groups.google.com/forum/#!searchin/logstash-users/sensu/logstash-users/pZkBl7XoTEg/pfCiErPxRI4J

It probably would not need specific codec, if the existing rabbitmq plugin supported customizing the payload.