Support for custom properties?

[Exulansis]

The SGL documentation states that custom properties can be used to describe rules for granting privileges based on attributes different than, e.g. the role of a principal.

Looking at the current implementation it seems like the JS package does not support this use case. I have forked and extended this library to support arbitrary custom properties (in a fairly naive way I believe).

I was generally curious if support for this feature will be added to this library, as well as what things need to be taken into consideration to do so robustly.

Thank you!