allow fragments on query-specified interfaces

I'm doing some queries on the introspection system for the purpose of generating documentation. It [routinely specifies](https://facebook.github.io/graphql/draft/#sec-Schema-Introspection) `name`, `description`, `isDeprecated` etc for many fields. I'd like to be able to do something like:

```graphql
interface Annotated {
name
description
isDeprecated
}

fragment Annotation on Annotated {
name
description
isDeprecated
}

query {
__schema {
types {
...Annotation
fields {
...Annotation
}
}
}
}
```