Allowed circular dependencies

[pie6k]

Introducing circular dependencies.

@ObjectType()
class A {
  @Field({type: () => B}) b: any; // note that thunk - `() => B` is now required in case of circular references istead of just `{type: B}`
}

@ObjectType()
class B {
  @Field({type: () => A}) a: any; 
}

Read more: graphql/graphql-js#373

Here is original approach I've got idea from: VojtaStanek@6ace1fb

[tibawatanabe]

Closing this PR in favour of #68 (which resolves the same problem)