ntopng: new package

ntopng is a high-speed web-based traffic analysis and flow collection
tool. ntopng => ntop next generation.

http://www.ntop.org/products/ntop/
nixos: add ntopng service

ntopng is a high-speed web-based traffic analysis and flow collection
tool. Enable it by adding this to configuration.nix:

  services.ntopng.enable = true;

Open a browser at http://localhost:3000 and login with the default
username/password: admin/admin.