Install direnv and store allowed directories

- Direnv lets us load environments when we enter a directory
- Direnv files need to be explicitly allowed for the first time; I don't want to
  have to `direnv allow` everything on every reboot
diff --git a/modules/direnv.nix b/modules/direnv.nix
new file mode 100644
index 0000000..eeec5aa
--- /dev/null
+++ b/modules/direnv.nix
@@ -0,0 +1,8 @@
+{username, ...}: {
+  home.programs.direnv = {
+    enable = true;
+    nix-direnv.enable = true;
+  };
+
+  config.environment.persistence."/nix/persist".users.${username}.directories = [".local/share/direnv/allow/"];
+}