nix commands

[rsteube]

Add nix commmands.

• [x] nix
• [x] nix-build
• [x] nix-channel
• [ ] nix-collect-garbage
• [ ] nix-common-options
• [ ] nix-copy-closure
• [ ] nix-env
• [ ] nix-hash
• [ ] nix-install-package
• [x] nix-instantiate
• [ ] nix-prefetch-url
• [ ] nix-push
• [x] nix-shell
• [ ] nix-store
• [ ] nixops
• [ ] nixos-build-vms
• [ ] nixos-container
• [ ] nixos-generate-config
• [ ] nixos-install
• [ ] nixos-option
• [x] nixos-rebuild
• [ ] nixos-version

https://github.com/hedning/nix-bash-completions https://github.com/spwhitt/nix-zsh-completions