Added ability to configure the queue_name_prefix delimiter to ActiveJob::QueueName.

@seuros @cristianbica @jeremy,

I'm currently trying to update my project to use ActiveJob for delayed work, but we use queue names following an `amqp.appname.queue` format, which is incompatible with ActiveJob's current implementation of `queue_name_from_part` (hard coded '_' delimiter).  This PR adds the ability to define the prefix delimiter on a per application or per class basis.

- Added `class_attribute :queue_name_prefix_delimiter` to `ActiveJob::QueueName` to allow for developers using ActiveJob to change the delimiter.
- Added tests to ensure custom delimiters operate properly.
- Updated source guide to include a blurb about the prefix delimiter.


Should there be `default_queue_name_prefix` too?