Add support for development with Nix

This patch adds support for development with the Nix package manager. In
order to support the traditional nix-shell tool as well as the (still
experimental) Nix Flakes feature of the upcoming version of Nix, this
patch adds shell.nix *and* flake.nix/flake.lock.  Usage instructions
have been added to the README.

flake.nix

@@ -0,0 +1,8 @@
+{
+  inputs = { flake-utils.url = "github:numtide/flake-utils"; };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in { devShell = import ./shell.nix { inherit pkgs; }; });
+}