Added #link into text.rb + tests

Added a #link method to text.rb since @sandal said in #689 (last comment) that a method was needed. Added tests too. Since this is my first PR in this repo, I'd love if any of you could tell me if I did anything wrong, so I can fix it. Thanks :smiley: 
Do you think it should return anything? This is sort of fire and forget method. Also by default it would return `nil` which is falsey, too.