nix-prefetch-url: Add --executable flag

pkgs.fetchurl supports an executable argument, which is especially nice
when downloading a large executable. This patch adds the same option to
nix-prefetch-url.

I have tested this to work on the simple case of prefetching a little
executable:

1. nix-prefetch-url --executable https://my/little/script
2. Paste the hash into a pkgs.fetchurl-based package, script-pkg.nix
3. Delete the output from the store to avoid any misidentified artifacts
4. Realise the package script-pkg.nix
5. Run the executable

I repeated the above while using --name, as well.

I suspect --executable would have no meaningful effect if combined with
--unpack, but I have not tried it.