v2.1.18

Typed custom props (#22)

Added schema, schema validation for `custom_properties`. Must be objects with `{_type: <X>, _value: <Y>}` where `<X>` is a JSON type or a custom string, and `<Y>` is:
- If `<X>` is a JSON primitive, `<Y>` is a literal of that type
- If `<X>` is `"object"`, `<Y>` is the same type as the base object, i.e. with property values being `{_type: <A>, _value: <B>}` objects
- If `<X>` is `"array"`, `<Y>` is an array of any of `{_type: <A>, _value: <B>}` objects
- Otherwise, `<Y>` can be any of the above types