Explore possibility of generic types

As projects like Relay have shown, it's relatively common to repeat the same generic structures of types multiple times within a project. In the case of Relay, I'm talking about Connections.

The GraphQL definition language already has explicit support for one particular form of generic type, arrays:

```graphql
type Foo {
id: ID!
bars: [Bar]
}
```

I'd like to start discussion about being able to do something similar for user-defined structures:

```graphql
generic ConnectionEdge {
node: T
cursor: String
}

generic Connection {
edges: ConnectionEdge
pageInfo: PageInfo
}

type Foo {
id: ID!
bars: Connection
}
```

The overall goal is to reduce the amount of boilerplate in creating schemas with repetitive structures.