adding new option.altDateField to specify a custom field to pull the date from using getdate.

When you need to pull the date from a field that is not pre-configure:
    // try to get date from one of the fields
    parser.once('pubdate', getdate);
    parser.once('lastbuilddate', getdate);
    parser.once('updated', getdate);

This option allows you to specify a custom field to get the date from:
    if (self.options.altDateField) {
      parser.once(self.options.altDateField, getdate)
    }