Check number of consumers for a specified queue

Hi,

Thanks for writing these plugins.

Just wondered if you'd accept this change to check the number of consumers for a given queue in check_rabbitmq_queue?

The arguments are backwards compatible but it's now possible to specifiy a fourth threshold for consumers or indeed to only specific 1 threshold if you only want to check total messages. The plugin output does always include "consumers" now but I don't think that should be a problem for existing implementation.

Thanks in advance.

Dan