Allow nix-shell to be used as a #! interpreter

This allows scripts to fetch their own dependencies via nix-shell. For
instance, here is a Haskell script that, when executed, pulls in GHC
and the HTTP package:

  #! /usr/bin/env nix-shell
  #! nix-shell -i runghc -p haskellPackages.ghc haskellPackages.HTTP

  import Network.HTTP

  main = do
    resp <- Network.HTTP.simpleHTTP (getRequest "http://nixos.org/")
    body <- getResponseBody resp
    print (take 100 body)

Or a Perl script that pulls in Perl and some CPAN packages:

  #! /usr/bin/env nix-shell
  #! nix-shell -i perl -p perl perlPackages.HTMLTokeParserSimple perlPackages.LWP

  use HTML::TokeParser::Simple;

  my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');

  while (my $token = $p->get_tag("a")) {
      my $href = $token->get_attr("href");
      print "$href\n" if $href;
  }

Note that the options to nix-shell must be given on a separate line
that starts with the magic string ‘#! nix-shell’. This is because
‘env’ does not allow passing arguments to an interpreter directly.

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