add --no-helper to suppress default.nix generation

I use npm2nix to generate npm-deps.nix, which I import as part of my app's .nix expression. I don't actually want a `default.nix` generated, so I've added an `--no-helper` flag to suppress that.

It might be clearer to require `--generate-helper` or something for those who want the helper, but if that's more common then it makes sense to leave it as-is, requiring an additional flag to disable it.