Provide means to add extra overlays to devhsell (#33)

* Provide means to add extra overlays to devhsell

* fmt

Author: David Arnold

default.nix

@@ -1,7 +1,8 @@
 { nixpkgs ? import ./nix/nixpkgs.nix
 , system ? builtins.currentSystem
+, overlays ? [ ]
 }:
 import nixpkgs {
   inherit system;
-  overlays = [ (import ./overlay.nix) ];
+  overlays = [ (import ./overlay.nix) ] ++ overlays;
 }

mkDevShell/options.nix

@@ -227,7 +227,8 @@ in
     ];
 
     packages =
-      builtins.filter (x: x != null)
+      builtins.filter
+        (x: x != null)
         (map (x: x.package) config.commands);
   };
 }