Allow purely additional schemas without needing to re-supply all core schemas

[apsillers]

I can supply an alternate schema set to validate_xml with the schemas argument:

validate_xml(file, schemas="/my/complete/schema/for/stix_1.x")

However, what I'm really interested in doing is supplying some additional schemas for STIX extensions. I don't want to (and don't want to be required to) supply the complete schema set for core STIX types.

Is there any technical obstacle to adding another argument (e.g., extension_schemas) that will still pull in the normal core schemas, but then augment it with some extra schema set?

[gtback]

Taking a look at stix-validator/sdv/validators/xml_schema.py, it doesn't seem impossible to add an extra_schemas argument. I'm not exactly sure how all of the rest of the code in that file is working, though.