utils.copyFromXML extended for output customFields property value

[kristofzerbe]

With this change, users will be able to reference attributes from customFields by appending .<property> to the FROM part of an item.

Example:

let parser = new RSSParser({
  customFields: {
    item: [
      ['media:thumbnail.url', 'mediaThumbnailUrl']
    ]
  }
});

[gltched-usr]

Any updates on this?
(+1)