Initialize //blog with lorri

To create new posts, I run...
```shell
hugo new name-of-post.md
```

While writing posts, I run...
```shell
hugo serve -D
```

I need hugo available on PATH environment variable, which lorri provides.

blog/.envrc

@@ -0,0 +1,2 @@
+source_up
+eval "$(lorri direnv)"

blog/shell.nix

@@ -0,0 +1,8 @@
+let
+  pkgs = import <nixpkgs> {};
+in
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    hugo
+  ];
+}