Ability to filter enqueued jobs on a particular queue

Hi there,

This pull request creates a new optional "Queue" configuration option, which allows the "Enqueued" count to be filtered for a specific queue.  I need this so I can configure this instance of the plugin to monitor for a specific queue which should always be below a certain threshold.

Later, it might be useful to add the ability to explicitly list multiple queues via  comma-separated queue names, then aggregate the counts across all, but for now that's overkill for me.

Cheers,
Adam