Finish modularising config

There are still some things I want to change, but at least there aren't two systems now.

modules/hledger.nix

@@ -0,0 +1,32 @@
+{ config, lib, pkgs, ... }:
+
+let
+  hledger-repo = pkgs.fetchFromGitHub {
+    owner = "chvp";
+    repo = "hledger";
+    rev = "feature/gain-reports";
+    sha256 = "07qsrq71pnkys11q6k2zc20xc9l3yp8dhzp1ar5bnkgcwbm69rcx";
+  };
+in
+{
+  options.chvp.hledger.enable = lib.mkOption {
+    default = false;
+    example = true;
+  };
+
+  config = lib.mkIf config.chvp.hledger.enable {
+    nixpkgs.overlays = [
+      (self: super: {
+        haskellPackages = super.haskellPackages.override {
+          overrides = hself: hsuper: rec {
+            hledger = hsuper.callCabal2nixWithOptions "hledger" hledger-repo "--subpath hledger" { };
+            hledger-lib = hsuper.callCabal2nixWithOptions "hledger-lib" hledger-repo "--subpath hledger-lib" { };
+          };
+        };
+      })
+    ];
+    home-manager.users.charlotte = { ... }: {
+      home.packages = [ pkgs.hledger ];
+    };
+  };
+}