Authenticate requests

[devgianlu]

This modification adds an authHandler options to authenticate requests.

const nuts = Nuts({
  repository: "<repo name>",
  token: "<github token>",
  authHandler: function (req, version) {
    // Return whether the requesting user has permission to retrieve this release
  }
});

authHandler must return true/false or a promise resolving to true/false.

[loprima-l]

Hi, I merged the project to a new repo to start maintain it, I would be glad if you can put your pull request here : https://github.com/loprima-l/nuts-2