add default.nix for installation via nix

Allows installation via nix from the repository itself
on NixOS and other Linux distribution that have Nix (i.e. Archlinux/Debian).

```
$ nix-build
$ ./result/bin/kconfig-hardened-check
$ nix-env -f . -i
```

It also provides an development environment for `nix-shell` with setuptools and
python in path

```
$ nix-shell
```

default.nix

+{ pkgs ? (import <nixpkgs> {}) }:
+with pkgs;
+
+pkgs.python3.pkgs.buildPythonPackage {
+  name = "kconfig-hardend-check";
+  src = ./.;
+  SOURCE_DATE_EPOCH = "1523278946";
+}