feat: extend the logic to VKontakte

[msramalho]

To do so, look at existing parsers and understand where the post's datetime is present on the VKontakte website. Modify:

• manifest.json - new content script for VKontakte
• vkontakte.js parser file
• if relevant see also #4
• update the README accordingly

any additional css/js changes.

Please include screenshots/example URLs in any PR.