Allow storing extra information not handled by the python domain model

[koenedaele]

The https://skosprovider.readthedocs.io/en/latest/api.html#module-skosprovider.skos SKOS domain model in Python maps input to an interface and throws away the rest. This means that anything not adhering to that interface is lost. However, it would be useful to hang on to that information and be able to pass it out again, even if a typical Python programmer won't do much with it.

• Create attributes for extra data on concept, collection and conceptscheme classes. Pick a good name.
• Decide on a format for those attributes. JSON-LD seems like the best fit since pyld is already a requirement for skosprovider and it feels very friendly to most developers
• Decide on the interaction between the regular domain attributes and what's in the extra data attribute. Having data be present in both place will probably create weird stuff.

Since this is an addition to the domain model, all skosprovider implementations will need to be updated. Worst case scenario this attrivute returns an empty JSON-LD doc. If it can be made backwards compatible with a good default value, it can go in a 1.3 release, otherwise a 2.0 release