Support interfaces

[natemcmaster]

Generate interfaces in the schema where appropriate.
http://graphql.org/docs/api-reference-type-system/#graphqlinterfacetype

class GraphQLInterfaceType {
  constructor(config: GraphQLInterfaceTypeConfig)
}

type GraphQLInterfaceTypeConfig = {
  name: string,
  fields: GraphQLFieldConfigMapThunk | GraphQLFieldConfigMap,
  resolveType?: (value: any, info?: GraphQLResolveInfo) => ?GraphQLObjectType,
  description?: ?string
};