logging controls on a per-worker basis

Refactored as per our previous discussions. Is there anything you'd like me to add to to the wiki/readme for how to use this functionality?

Also, would monkeypatching be the best way to inject into Sidekiq::Extensions::DelayedClass?

```ruby
module Sidekiq
  module Extensions
    class DelayedClass
      sidekiq_logger do |logger|
        logger.level = Logger::WARN
        logger.execution_threshold = 5
      end
    end
  end
end
```