Add nix shell build environments

[jb55]

This enables a build environment using nix. Should work on most linux varieties but I've only tested on NixOS.

[hoytech]

Thanks!

[tim-tx]

I wrote a barebones NixOS package & module for this too: https://gist.github.com/tim-tx/7eb77fe78f419d0873d4b459464c6f63