lib/utils/config/index.js/set
function set(key, value) {
  let config = getGlobalConfig();
  if (key && typeof key === 'string' && typeof value !== 'undefined') {
    config = _.set(config, key, value);
  } else if (_.isObject(key)) {
    config = _.merge(config, key);
  } else if (typeof value !== 'undefined') {
    config = _.merge(config, value);
  }
  // update config meta
  config.meta = config.meta || {};
  config.meta.updated_at = Math.round(+new Date() / 1000);
  // write to .serverlessrc file
  writeFileAtomic.sync(serverlessrcPath, JSON.stringify(config, null, 2));
  return config;
}