Add twitter:site meta tag nodes

[harrydayexe]

Hi all, This change is quite simple but something that is quite nice to have.

The tag twitter:site is used to identify the site by it's Twitter handle (for example the @ SwiftBySundell twitter account) when shown in a card. See the (documentation)[https://developer.twitter.com/en/docs/twitter-for-websites/cards/overview/markup]

I added a new node .twitterUsername(_) to set the Twitter username to be associated with the page.