Specify opaque types in meta data
In the meta section i can state whether to allow opaque types with the `ks-opaque-types` key.

I think it would be a good idea to allow the specification of witch opaque types are allowed instead of just `true`.

This allow someone to easily see witch types are needed and to produce an error at generation time if an extra type is used (or if there is a typo in a type name) 