Conditional resolving / optional expanding

It would be nice if you could somehow make resolving conditional whether i request subfieds at all:

ExampleData:

```
vehicleData = [
{
licenseplate: "AB-CD123",
customerNo: "1234"
},
...
]

customerData = [
{
CusteormNo: "1234",
Name: "John Doe"
}
]

```

How it is
```
schemas:

type vehicle {
licenseplate: String
CustomerNo: String
Customer: Customer
}

type Customer {
CustomerNo: String
Name: String
}

Resolver:

Vehicle: {
Customer: ({ CustomerNo }, args, context, info) => {
return customers.find( x => x.CustomerNo == CustomerNo )
},
},
```

```
query for vehicle without customer:

query ExampleQuery {
vehicles{
licenseplate
CustomerNo
}
}

query for vehicle with customer:

query ExampleQuery {
vehicles{
licenseplate
Customer {
CustomerNo
Name
}
}
}

```

How i wish it would be

```
schemas:

type vehicle {
licenseplate: String
Customer: String | Customer <- dynamic fields?
}

type Customer {
CustomerNo: String
Name: String
}

Resolver:

Vehicle: {
Customer: ({ Customer }, args, context, info) => {
if( _noSubfields_ ) return Customer; /* no expensive resolving */
return customers.find( x => x.Customer== Customer)
},
},
```

```
query for vehicle without customer:

query ExampleQuery {
vehicles{
licenseplate
Customer
}
}

query for vehicle with customer:

query ExampleQuery {
vehicles{
licenseplate
Customer {
CustomerNo
Name
}
}
}

```