code: function make_post_data (req) {
    return querystring.stringify({
      client_id: process.env.GITHUB_CLIENT_ID,
      client_secret: process.env.GITHUB_CLIENT_SECRET,
      code: req.query.code,
      state: req.query.state
    });
  }
docs: nodejs.org/api/querystring.html 
  make_post_data simply extracts the code and state (hash) from the request
  and combines those data with the GitHub Client Id & Secret as a query string
  @param {Object} req - the request object sent by GitHub with code & state
  @returns {Object} the stringified object required to request OAuth2 Token