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.

shell.nix

@@ -0,0 +1,17 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.mkShell {
+  shellHook = ''
+    export PKG_CONFIG_PATH=${pkgs.openssl.dev}/lib/pkgconfig
+  '';
+  buildInputs = with pkgs; [
+    crystal
+    lucky-cli
+    overmind
+    nodejs
+    openssl.dev
+    postgresql
+    shards
+    yarn
+  ];
+}