Add support for notifiers which works on collections

[davidwessman]

Hello!

I would really like a raise_all mode when running Bullet in tests.

Instead of just raising the first error, I want it to raise all of them at once.
I cannot see how this could be implemented with the current pattern of the notifiers.

Currently we are using a custom bullet_teardown after each test and monkey-patched the Bullet::Rack middleware to combine all errors the same way:

def bullet_teardown
  return unless Bullet.start?

  Bullet.perform_out_of_channel_notifications if Bullet.notification?
  if Bullet.warnings.present?
    warnings = "[Bullet warnings]\n"
    warnings += Bullet.warnings
      .values.flatten
      .map do |warning|
        "#{warning.class.name}\n#{warning.body_with_caller}"
      end.join("\n-----\n\n")

    raise(StandardError, warnings)
  end
ensure
  Bullet.end_request
  Bullet.enable = false
  Bullet.reset_safelist
end