emacs: add extraConfig option

modules/programs/emacs.nix

@@ -13,6 +13,12 @@ let
 
   emacsWithPackages = emacsPackages.emacsWithPackages;
 
+  createConfPackage = epkgs:
+    epkgs.trivialBuild {
+      pname = "default";
+      src = pkgs.writeText "default.el" cfg.extraConfig;
+    };
+
 in {
   meta.maintainers = [ maintainers.rycee ];
 
@@ -28,6 +34,23 @@ in {
         description = "The Emacs package to use.";
       };
 
+      # NOTE: The config is placed in default.el instead of ~/.emacs.d so that
+      # it won't conflict with Emacs configuration frameworks. Users of these
+      # frameworks would still benefit from this option as it would easily allow
+      # them to have Nix-computed paths in their configuration.
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        example = ''
+          (setq standard-indent 2)
+        '';
+        description = ''
+          Configuration to include in the Emacs default init file. See
+          <link xlink:href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Init-File.html"/>
+          for more.
+        '';
+      };
+
       extraPackages = mkOption {
         default = self: [ ];
         type = hm.types.selectorFunction;
@@ -68,6 +91,10 @@ in {
 
   config = mkIf cfg.enable {
     home.packages = [ cfg.finalPackage ];
-    programs.emacs.finalPackage = emacsWithPackages cfg.extraPackages;
+    programs.emacs = {
+      finalPackage = emacsWithPackages cfg.extraPackages;
+      extraPackages = epkgs:
+        optional (cfg.extraConfig != "") (createConfPackage epkgs);
+    };
   };
 }