Things done

• added varnish/bin to the exporter's PATH as varnishstat is expected to be in it
• enabled the tests
• added myself as maintainer
• extended the module to expose most of the exporters options
• intentionally did not expose the "-N" flag as I did not find any reference to it in varnishstat and because it could still be used with extraFlags

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

/cc @MostAwesomeDude