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 } }