Read env variables using envy library

Using my dear friend's, dmjio's, excellent library, envy -- to read and parse
variables from the system environment.

I added and git-ignored the .envrc file that contains API secrets. I'm using
Envy to read these values, so that I don't hard-code these values into the
source code.

shell.nix

@@ -10,6 +10,7 @@ in pkgs.mkShell {
       hpkgs.warp
       hpkgs.cryptonite
       hpkgs.uuid
+      hpkgs.envy
     ]))
   ];
 }