Introduce Bulk deliveries

[manojmj92]

Introduces bulk deliveries mentioned in #89

Usage

class CommentNotification < Noticed::Base
  deliver_by :database
  deliver_by :postmark
  deliver_by :postmark, bulk: { group_size: 1000 }
end

class DeliveryMethods::Postmark < Noticed::DeliveryMethods::Base
 # A custom delivery method can deliver both individually and in bulk
 # For individual delivery, use `deliver` and for bulk delivery use `bulk_deliver`

  def deliver
     # you will have access to a single`recipient` here

     post "https://api.postmarkapp.com/email/send", {
        to: recipient.email,
        subject: 'Something'
      }
  end

  def bulk_deliver
     # you will have access to one batch of `recipients` here

       post "https://api.postmarkapp.com/email/send", {
         to: recipients.map(&:email),
         subject: 'Something'
       }
  end
end

User.all.count
# => 5000

CommentNotification.deliver_later(User.all)
# => Enqueues DeliveryMethods::Database.deliver 5,000 times.
# => Enqueues DeliveryMethods::PostMark.deliver 5,000 times.
# => Enqueues DeliveryMethods::Postmark.bulk_deliver 5 times.

[manojmj92]

@excid3 this is ready for review. Documentation isn't done yet, I will add them if you are ok with the approach 🙂

[madhums]

@excid3 Could you review this? Seems like an essential feature!