Proposal: Add Interface inheritance for definitions

[SerafimArts]

Spec (http://facebook.github.io/graphql/#sec-Interfaces) does not proposes this feature. But this is will be good for ISP (Interface Segregation Principle) as example.

Example:

Base definitions

interface Creatable {
    createdAt: String!
}

interface Updatable {
    updatedAt: String
}

Variation of implementation

interface Person extends Creatable, Updatable {
    id: ID!
    name: String!
    # ...
}

class User implements Person {
    # Impl of interface fields: 'id', 'name', 'createdAt', 'updatedAt'
}

Interface fields overriding

Discussion needed. I think. e.g. how to consider the LSP (Liskov Substitution Principle)?

1. Allows NonNull for Nullable fields?
2. Allows ListType for "Single" type def?
3. Allow to increase/decrease the number of arguments for field input?

# Example
interface HasDates extends Creatable, Updatable {
    updatedAt: String! # Overriding nullable field
}

Errors

1. Error while cyclic inheritance

interface A extends B { ... }
interface B extends A { ... }

2. Error inheritance twice

interface Person extends Creatable, Updatable { ... }

type User implements Person, Creatable {
    # Error: Can not implement "Creatable" interface. Interface already defined in Person.
}

Like everything foreseen, there are ideas? =)

[SerafimArts]

I think that this proposal should be moved here graphql/graphql-spec#335