ability to specify global queue for ActionMailer extension

[jjb]

Threw this together just to get feedback.

If you like the idea of this feature I will write tests and improve the code.

(clearly default_options will not be named that since it's generic to ActionMailer after being mixed in, and also I'll do something about the repeated code)

[mperham]

It's easy to reopen the class in your initializer and use sidekiq_options. WDYT?

[jjb]

Okay, sounds good.

For anyone coming across this in the future wanting the same thing, here's how:

# config/initializers/sidekiq.rb
module Sidekiq
  module Extensions
    class DelayedMailer
      sidekiq_options queue: "mailer"
    end
  end
end