feat: Add transform function to selector

[saar]

If the array includes a callback function as 3rd element, all the results pass through that transformer. Returned values are the result of selector.
For example:

date_published: {
    selectors: [[
        'meta[name="article:published_time"]',
         'value',
          (item) => moment.from(convertNumbersToEnglish(item), 'fa',
            'YYYY/MM/DD - HH:mm').toISOString();
    ]]
}

extend: {
  tags: {
    selectors: [[
        'meta[name=\'article:tag\']', 'value', (item) => {
        return item.split(/\s*,\s*/);
      }]],
    allowMultiple: true
  }
}