nix dev-shell: Add --command option
nix dev-shell: Add --command option

Note: like 'nix run', and unlike 'nix-shell', this takes an argv
vector rather than a shell command. So

  nix dev-shell -c 'echo $PATH'

doesn't work. Instead you need to do

  nix dev-shell -c bash -c 'echo $PATH'

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix