More readable queue configuration

[freysie]

It would be super cool instead of having to write

  config.default_queues = %w[
    transcoding:5:1000:5
    mailers:1:1000:5
  ]

if I instead could write

  config.default_queues = {
    transcoding: { threads: 5, garbage_limit: 1000, max_retries: 5 },
    mailers: { threads: 1, garbage_limit: 1000, max_retries: 5 }
  }

or something like that.

(Not sure about the naming. All of three options—even the threads one—are really upper limits, aren't they?)