Re-introduce flake support

Relevant issue: #24

README.md

@@ -21,6 +21,12 @@ To run the current development version:
 nix-shell -p '(import (builtins.fetchTarball "https://github.com/utdemir/nix-tree/archive/main.tar.gz") {}).nix-tree' --run nix-tree
 ```
 
+Or, if you use a Nix version with flake support:
+
+```
+nix run github:utdemir/nix-tree
+```
+
 ## Usage
 
 ```

flake.lock

@@ -0,0 +1,44 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1623875721,
+        "narHash": "sha256-A8BU7bjS5GirpAUv4QA+QnJ4CceLHkcXdRp4xITDB0s=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "f7e004a55b120c02ecb6219596820fcd32ca8772",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "ref": "master",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1623176226,
+        "narHash": "sha256-54a9uvHlIlK3i0b36HfGMc4zqM0BpMOOiFYBxEhQFK8=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "51bb9f3e9ab6161a3bf7746e20b955712cef618b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,25 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils/master";
+  };
+
+  description = "Interactively browse the dependency graph of your Nix derivations.";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    let
+      overlay = self: super: {
+        nix-tree =
+          (self.callPackage ./default.nix { }).nix-tree;
+      };
+    in
+    { inherit overlay; } // flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; overlays = [ overlay ]; };
+      in
+      {
+        defaultPackage = pkgs.nix-tree;
+        devShell = (pkgs.callPackage ./default.nix { }).shell;
+      }
+    );
+}