Directive composition

[JoviDeCroock]

Idea

Related to #146 I'd like to extend our traverser to have a certain set of directives like the one proposed in the issue @extra. When passed to the cacheExchange it would automatically stop at these directives, strip them out and execute a callback allowing you to return an altered operation, ....

This would for example allow you to add more variables, allow the populateExchange to become a HOC passed into this object.

Rough example

cacheExchange({
  directives: {
    extra: (op, directiveArgs) => ({ ....op, variables: { ...op.variables, ...directiveArgs }}) 
  }
})

This is very rough and I'll most likely alter this when hacking on it but I would like to send this in to get some more idea's

[andyrichardson]

I think the order importance of exchanges is a bit of a pain but I'm not sure nesting them inside the cache is the way to go.

Are there any specific decorators that can't be resolved before/after the cache?

[JoviDeCroock]

Are there any specific decorators that can't be resolved before/after the cache?

No but I'm trying to find a middle ground on this. Much like for instance redux has middlewares it could be used to make our cache smarter (and we need to solve the issue anyway so I thought let's look for a very extendable solution)

[kitten]

I think it's sufficient to fix #146 in isolation :)

[JoviDeCroock]

@kitten do you agree with @andyrichardson to catch a directive like extra in a separate exchange?