added 'rejections' to the default property exposer

[DiniFarb]

Referencing to Issue #1955

[DiniFarb]

Or maybe it is better to expose the rejections like the exception handler.

Something like this:

/**
 * Define getter for `rejections` which exposes the default `rejectionHandler`
 * @type {Object}
 */
Object.defineProperty(winston, 'rejections', {
  get() {
    return defaultLogger.rejections;
  }
});

[wbt]

Closing in favor of #2021 which has been merged in awaiting release. If you don't think that covers it, please comment for reopening!