Add support for passing extra env variables

[edofic]

Bascause some funky setup.py files take arguments from the environment.
As far as I understand pypi2nix runs a pure nix-shell exactly to avoid variables like this so I created a new flag to explicitly pass settings in.

---

This change is 

[garbas]

@edofic sorry sorry sorry ... this PR completely skipped my radar. will look it over the next 2 days.

it looks ok but i would like to test it and maybe create a small example which we can use as a test

[garbas]

hmm might be also cool to be able to evaluate nix for extra_env. so that you could do something like: NIX_CFLAGS_COMPILE="-I${pkgs.cyrus_sasl.dev}/include/sasl" for python-ldap package