lib: create 'my' extensions

Inspired by this [1] repo, I want to add some functions to `lib.my`. I
would have liked to make it work without having to use an overlay, but I
did not manage to do it...

[1]: https://github.com/hlissner/dotfiles

lib/attrs.nix

@@ -0,0 +1,7 @@
+{ lib, ... }:
+let
+  inherit (lib) filterAttrs mapAttrs';
+in
+{
+  mapFilterAttrs = pred: f: attrs: filterAttrs pred (mapAttrs' f attrs);
+}