Support for custom key/value in types and properties

[lvca]

Discussed in #168

^{Originally posted by PhantomYdn October 25, 2021}
It will be nice to have:

1. DocumentType.getCustom(String key), DocumentType.setCustom(String key, String value), Property.getCustom(String key), Property.setCustom(String key, String value) to be able to save additional metainformation in the schema.

[lvca]

This issue was requested by @PhantomYdn.

[PhantomYdn]

Thank you, @lvca ! It was very fast:)