[WIP] - Organizers

[totallymike]

So I've been thinking about interactors lately, and how they can operate together. Organizers have been on my wish list for a while, and I had direct use for an organizer, so I implemented it 😄

This is still in a bit of a transition state, but we're dogfooding it so learning about its edge cases quickly.

Even though it's not fully baked, I thought I'd put it up here to get some feedback on it. Feel free to examine at your leisure.

Some things I want to address before I feel this is ready for primetime:

• I don't really like specs around the Organizer. Error handling feels loose and unpredictable.
• README.md needs to talk about Organizers
• The Interactor.Organizer module needs @moduledoc, @docs, and doctests
• This requires that all organized interactors return {:ok, context}, which is a dealbreaker.
  It works for us because we're lining up HTTPoison requests, but it won't work for, say, Ecto.
• It catches all exceptions and tries to rollback if any are caught. That seems fine, but it also doesn't do a good job of reporting the exception.
• It also seems to clobber the properties of the exception if it happens far enough down the chain. This I want to figure out and test. I think it has to due with the way execute_interactors tries to handle varying tuple sizes in its error handling.
• Actually test this with Ecto.
• I'd also like to put together a better story for rolling up the context.

[alanpeabody]

@totallymike do you think the ideas I sketched out in #13 would serve the same purpose as organizers?

[totallymike]

Yes that does address this quite neatly. And then some. I'll move related discussion to that issue.

I'll leave this open for reference for a while longer, but we can close it once #13 is a bit more fleshed out.