Support setting the seed externally, as an environment variable

[marcinwyszynski]

It may be an extremely rare occurrence, but sometimes it could be useful to be able to predict the value of the null resource ID. Being able to supply a custom seed could sometimes make a difference.

Please let me know if there's a better way to do it. I don't think exposing that as a public API would make sense, but someone who's in search of a solution is likely to find this little escape hatch.