Nix support

[srid]

Adding a default.nix would help users on NixOS, or using Nix, to easily build himalaya. Here's the default.nix that I used to build the current master branch:

let 
  pkgs = import <nixpkgs> {};
in 
pkgs.rustPlatform.buildRustPackage rec {
  pname = "himalaya";
  version = "0.2.2";

  src = ./.;

  nativeBuildInputs = with pkgs; [
    pkg-config 
    openssl.dev
  ];
  PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";

  cargoSha256 = "sha256-i8YKE5Z2xoNyjFktRRVA45zD6Qa0aPNC0AOjHUIglgk=";

  meta = with pkgs.stdenv.lib; {
    description = "Minimalist CLI email client";
    homepage = "https://github.com/soywod/himalaya";
  };
}

[soywod]

Thanks a lot!! Maybe consider opening a PR instead? So you will be part of the contributors ;)