Added ability to programatically specify custom queue on FileAdder

[chrispage1]

We've run into an issue where we have thousands of conversions being generated, it blocks up the queue for conversions that should be considered a higher priority.

This small addition allows us to programatically tap into the FileAdder and set a custom queue depending on the job we're performing.

So we can have two queues, one called conversions and one called high_priority_conversions for example.

[freekmurze]

Thanks!