code: function stringifyLoaders (loaders) {
      return loaders
        .map(
          obj =>
            obj && typeof obj === 'object' && typeof obj.loader === 'string'
              ? obj.loader +
                (obj.options ? '?' + JSON.stringify(obj.options) : '')
              : obj
        )
        .join('!')
    }
docs: stringify an Array of loader objects