Ejecting express-middleware to its own module

[mattecapu]

1. It doesn't really belong to the network layer. It's just a mean to support the "batching protocol" that should be separated from the rest.
2. Not everybody use express. I use koa and I made koa-graphql-batch to use this network layer, thus I don't need express-middleware.
3. If one doesn't bother to do properly minification/tree-shaking, the middleware code will end up client side where it's all but needed.

I could take care of this if @nodkz agrees.

[nodkz]

@mattecapu 👍 I agree with you. Express-middleware should be extracted from RRNL. In the next month I'll start implementing subscriptions and bump new major version.

Before this moment let keep untouched this extra 1.6 kb of express-middleware for semver compatibility.

[nodkz]

BTW. Please add a link to your koa-graphql-batch to readme. Thanks!