New option to specify additional configuration

This enables emacs configuration to refer nix store paths.

nix-integration.patch

@@ -0,0 +1,48 @@
+diff --git a/core/core.el b/core/core.el
+index cb12f8d08..03f370691 100644
+--- a/core/core.el
++++ b/core/core.el
+@@ -63,22 +63,21 @@ decrease this. If you experience stuttering, increase this.")
+   "The root directory for Doom's modules. Must end with a slash.")
+ 
+ (defconst doom-local-dir
+-  (if-let (localdir (getenv "DOOMLOCALDIR"))
+-      (expand-file-name (file-name-as-directory localdir))
+-    (concat doom-emacs-dir ".local/"))
++  (or (getenv "DOOMLOCALDIR")
++      "@local@/")
+   "Root directory for local storage.
+ 
+ Use this as a storage location for this system's installation of Doom Emacs.
+ These files should not be shared across systems. By default, it is used by
+ `doom-etc-dir' and `doom-cache-dir'. Must end with a slash.")
+ 
+-(defconst doom-etc-dir (concat doom-local-dir "etc/")
++(defconst doom-etc-dir "~/.local/doom"
+   "Directory for non-volatile local storage.
+ 
+ Use this for files that don't change much, like server binaries, external
+ dependencies or long-term shared data. Must end with a slash.")
+ 
+-(defconst doom-cache-dir (concat doom-local-dir "cache/")
++(defconst doom-cache-dir "~/.cache/doom/"
+   "Directory for volatile local storage.
+ 
+ Use this for files that change often, like cache files. Must end with a slash.")
+@@ -196,7 +195,7 @@ users).")
+ (setq abbrev-file-name             (concat doom-local-dir "abbrev.el")
+       async-byte-compile-log-file  (concat doom-etc-dir "async-bytecomp.log")
+       bookmark-default-file        (concat doom-etc-dir "bookmarks")
+-      custom-file                  (concat doom-private-dir "init.el")
++      custom-file                  (concat doom-etc-dir "init.el")
+       custom-theme-directory       (concat doom-private-dir "themes/")
+       desktop-dirname              (concat doom-etc-dir "desktop")
+       desktop-base-file-name       "autosave"
+@@ -489,6 +488,7 @@ to least)."
+                    (daemonp))
+                (file-exists-p doom-env-file))
+       (doom-load-envvars-file doom-env-file))
++    (load "nix-integration.el" t)
+ 
+     (require 'core-modules)
+     (let (;; `doom-autoload-file' tells Emacs where to load all its functions