Add success! to context

I like being able to call fail! and pass in my failure message, I would like to do the same with success!

``` ruby
class MarkOrderShipped
  include Interactor

  def perform
    if order.shipped_at.present?
      fail!(error: 'Order has already been marked as shipped')
    else
      order.shipped_at = Time.zone.now
      order.save!
      OrderMailer.ship_confirmation(order.id).deliver
      # context[:notice] = "#{order.number} marked as shipped, #{order.customer.email} has been notified."
      success!(notice: "#{order.number} marked as shipped, #{order.customer.email} has been notified.")
    end
  end
end
```