Allow interface type definition to hint about interface implementors

[vladar]

Now when interface implementor is not referenced anywhere in schema it remains invisible for GraphQL during validation phase.

As a workaround we allow types option on Schema to pass such implementors directly. But a better way for this could be adding implementors option to interface type definition that could act as a hint for schema.

This option will accept an array or thunk of types (similar to fields and interfaces options on ObjectType).

This mechanism will have advantages over current types option on Schema only when type definitions are initialized on demand (vs collected during schema creation as it is done now).

See #38 for more info

[vladar]

After more thinking: this is bad idea. Looking for better ways to solve original issue.