Add `nix-instantiate`-like command

[nhey]

Make store derivations more easily accessible (ie. without running morph build).

Maybe a more general command like nix eval is more useful. Derivation can be instantiated like so:

~/git/nixpkgs  (master)
$ nix eval -f . hello.drvPath
"/nix/store/3axqhq27f6hyj09rs6gf91rwdxlw2k7w-hello-2.10.drv"