Allow indent_size to be unset for shfmt (numtide#97)

* Allow indent_size to be unset for shfmt

So the defaults don’t get in the way of reading the config file.

* Revert the breaking change

Also add some explanation and simplify a conditional.

Author: sellout

programs/shfmt.nix

@@ -7,19 +7,24 @@ in
     enable = lib.mkEnableOption "shfmt";
     package = lib.mkPackageOption pkgs "shfmt" { };
     indent_size = lib.mkOption {
-      type = lib.types.int;
+      type = lib.types.nullOr lib.types.int;
       default = 2;
       example = 4;
       description = lib.mdDoc ''
-        Sets the number of spaces to be used in indentation. Uses tabs if set to zero.
+        Sets the number of spaces to be used in indentation. Uses tabs if set to
+        zero. If this is null, then [.editorconfig will be used to configure
+        shfmt](https://github.com/patrickvane/shfmt#description).
       '';
     };
   };
 
   config = lib.mkIf cfg.enable {
     settings.formatter.shfmt = {
       command = cfg.package;
-      options = [ "-i" (toString cfg.indent_size) "-s" "-w" ];
+      options =
+        (lib.optionals (!isNull cfg.indent_size)
+          [ "-i" (toString cfg.indent_size) ])
+        ++ [ "-s" "-w" ];
       includes = [ "*.sh" ];
     };
   };