query based on fields of complex variables

Complex variables are described in the spec https://facebook.github.io/graphql/#sec-Variables-Are-Input-Types

This example takes a ComplexInput and passes it to the findDog query:

```input ComplexInput { name: String, owner: String }

query takesComplexInput($complexInput: ComplexInput) {
findDog(complex: $complexInput) {
name
}
}
```

What I would really like to do is use specific fields in the ComplexInput variable like this:

```query takesComplexInput($complexInput: ComplexInput) {
findDogByName(name: $complexInput.name) {
name
}
}
```

This is not valid GraphQL, but it would simplify a feature we are working on quite a bit.

For context: Graphcool is a backend application platform that combines GraphQL and Lambda. Many of our features are based on automatically generated schemas and stored queries we execute on the server in response to some event, so our needs might differ significantly from the broader community.