customizable logic for preparing requests, parsing responses

[ariebovenberg]

encountered in #10

a.k.a. "middleware". Currently request preparation and response handling is implemented somewhat generally. However, most GraphQL servers provide some extra logic.

It should be possible for users to specify their own logic for this. For example:

• Adding specific headers to requests
• non-json serialization methods
• custom HTTP status code handling
• custom error handling

A quick solution would be to allow some kind of middleware= parameter in execute(). However, some thought needs to go into this to ensure the API is flexible enough.