Option to force-include certain types

**Is your feature request related to a problem? Please describe.**
Sometimes we may want to include certain type descriptions in the TSD even if there are no instances of them and even in MINIMAL mode. For example, in an annotation tool, we might want to always include all user-defined types (even if there are no instances) but no system-defined types.

**Describe the solution you'd like**
Add an option to the serializer to include certain types (and their transitive dependencies) even if there are no instances of that type in MINIMAL mode.