Support for Tuples

I work with a datatype, [mobiledoc cards](https://github.com/bustle/mobiledoc-kit/blob/master/MOBILEDOC.md), that's represented as a tuple of a string followed by an object. Currently, to make it work in graphql I need to denormalize it to an object, then normalize to a tuple, and so on back and forth between our backend and frontend. Ideally I'd like a way to represent that the data structure as a tuple.

I would like to be able to do the following
```
type CardPayload {
foo: String!
bar: Int!
}

type Content {
cards: [String!, CardPayload!]
}

query {
content {
cards [name, { foo }]
}
}
```