code: function plugin(options) {
      if (!options || typeof options !== 'object') {
          throw new Error('AjvMoment#plugin requires options');
      }
      if (!options.ajv) {
          throw new Error(`AjvMoment#plugin options requries an 'ajv' attribute (ajv instance)`);
      }
      if (!options.moment) {
          throw new Error(`AjvMoment#plugin options requries a 'moment' attribute (moment.js)`);
      }
      const { ajv, moment } = options;
      ajv.moment = moment;
      const keywordSettings = {
          type: 'string',
          statements: true,
          errors: true,
          inline
      };
      if (ajv) {
          ajv.addKeyword('moment', keywordSettings);
      }
      return keywordSettings;
  }
docs: Configure the plugin by attaching moment to the ajv instance and defining the
  'moment' custom keyword
  @param  {Object} options - plugin options
  @return {Object} keywordSettings