Add 'nix upgrade-nix' command

This command upgrades Nix to the latest stable version by installing a
store path obtained from

  https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix

which is the same store path that the installer at
https://nixos.org/nix/install.sh uses.

The upgrade fails if Nix is not installed in a profile (e.g. on NixOS,
or when installed outside of the Nix store).