Fix #1314
Fix #1314

Also, make nix-shell respect --option. (Previously it only passed it
along to nix-instantiate and nix-build.)

rycee/nix,peti/nix,layus/nix,vcunat/nix,pikajude/nix,peti/nix,vcunat/nix,rycee/nix,peti/nix,shlevy/nix,pikajude/nix,vcunat/nix,rycee/nix,NixOS/nix,dezgeg/nix,layus/nix,NixOS/nix,shlevy/nix,dezgeg/nix,dtzWill/nix,ehmry/nix,rycee/nix,dezgeg/nix,peti/nix,dezgeg/nix,ehmry/nix,layus/nix,zimbatm/nix,shlevy/nix,pikajude/nix,dtzWill/nix,ehmry/nix,NixOS/nix,dtzWill/nix,ehmry/nix,layus/nix,zimbatm/nix,zimbatm/nix,peti/nix,dtzWill/nix,vcunat/nix,layus/nix,shlevy/nix,dtzWill/nix,zimbatm/nix,rycee/nix,ehmry/nix,ehmry/nix,NixOS/nix,dezgeg/nix,shlevy/nix,NixOS/nix