add support for promising at organizer level

[markburns]

Adds support for

class SomeOrganizer
  include Interactify

  organize \
    DoStep1.promising(:step_1),
    DoStep2.promising(:step_2)

Example failure at load time

SomeOrganizer::DoStep1 does not promise:
step_1

Actual promises are:
step1

These calls to promising return the receiver. I.e. DoStep1 and DoStep2 in the example above, but also verify that the promises specified in the arguments :step_1 and :step_2 respectively, match the promises defined in each interactor.

Why add this?

It's for the reader of the code in the future.
By explicitly calling out the promises, it's easier to debug where something is added in a chain.
By also verifying at load time, we ensure that the promising calls do not drift from the interactor promises implementation, as they raise an exception.

I.e. this is enforced executable documentation.