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: 
Well, the other methods for adding text return false when the text is `nil`, so I figured this one should too.