Adding custom scalars

It would be useful if there would be a way to map certain primitive field scalars from certain types of the prisma schema to custom graphql scalars for example with [graphql-scalars](https://github.com/Urigo/graphql-scalars) to have an even stricter type system which also reduces the need for [class-validator](https://github.com/typestack/class-validator) since the validation takes place directly at the resolver level. It would be really nice to have this feature to have more control over the schema generation.

### Example:

type definition of `Person` in `prisma.schema` with the primitive type `String`:
``` prisma
model Person {
email String
}
```

type definition of `Person` in `schema.graphql` with a custom scalar `EmailAddrress`:
``` gql
type Person {
email: EmailAddress
}
```

generated type definition of `Person` as a typegraphql object type with custom scalar resolver:
``` ts
import { EmailAddressResolver } from 'graphql-scalars';

@ObjectType()
export class Person {
@Field(() => EmailAddressResolver)
email!: string;
}
```