nixos/pleroma: add pleroma NixOS module

This module is not trying to configure either postgresql nor nginx.
It's is a design decision, not an omission. A webserver setup can be
highly complex.

The idea is trying not to be smarter than the user, providing them
with a simple tool. They are smart enough to figure the interaction
between the various component by themselves!

The module has one and only job: setting up a pleroma service.

modules/pleroma.nix

@@ -8,7 +8,7 @@ in {
 
       package = mkOption {
         type = types.package;
-        default = import ../default.nix { inherit pkgs stdenv; };
+        default = import ../default.nix { inherit pkgs; };
         description = "Pleroma package to use.";
       };