Refactor into multiple packages

[mattecapu]

Following #30, I think this module could provide just its core feature (support client-side middlewares for a Relay Network Layer) and then provide the built-ins middlewares as separated packages.
It would be relatively straightforward to implement, improve architecture and bundle size.

I can take care of this too @nodkz

[nodkz]

I dislike an idea of many nano modules and polluting package.json. Almost all middlewares I'm using in my app.

But maybe core can be extracted to another package and all this middlewares keep here in one package.

Anyway I want to hear more thoughts...

[mattecapu]

Personally, I use only urlMiddleware in production. It kind of bothers me having to use tree-shaking to exclude the other middlewares.
It's the same philosophy of express, browserify, babel, and so on. The size of the project is smaller, but I think that as Relay grows this could become a focal package.
Also it would help with issue tracking and semver. Much more flexibility.