Include customSettings option to configure regreet (#4)

* Attempt to add regrett configuration
* Prevent double install of regreet
* Install regreet over program enable
* Force regreet settings
* Revert regreet over programs enable.
* Move regreet config into homemanger.

modules/conf.nix

@@ -88,7 +88,7 @@
         ];
         type = with lib.types; listOf attrs;
         description = ''
-          Adds modules to ironbar.
+          Adds modules to ironbar. See https://github.com/JakeStanger/ironbar/wiki/ for more information.
         '';
       };
     };