Promising coordinator

[mkj-is]

We have several implementation of guaranteeing/promising cvoordinator which is returning value during stop.

A subspec with dependency on PromiseKit was proposed and I personally agree this is a good idea and could help to add more PromiseKit related architecture components to FuntastyKit.

Example implementation on which can we build upon can be taken from here:

https://github.com/thefuntasty/kartel-ios/pull/40

[mikolasstuchlik]

Request for comments on my new implementation:

https://github.com/thefuntasty/nesnezeno-ios/blob/feature/nes-233-fotograph/Nesnezeno/Sources/Global/PromisingCoordinator.swift