Callbacks and deliver short circuiting

[jwoertink]

Fixes #65
Fixes #61

This PR allows you to setup before/after callbacks when sending an email as well as short circuiting an email from being delivered.

class FriendIsLiveEmail < BaseEmail
  def initialize(@recipient : Carbon::Emailable, @friend : User)
  end

  before_send do
    if !@recipient.wants_email_from_friend?(@friend)
      self.deliverable = false
    end
  end

  after_send do |response|
     MarkEmailAsRecentlySent.run(self, sent_at: Time.utc)
  end
end