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: 
I don't know how well we thought out this interface, but if right now we're returning `false` to signal non-rendering, we might as well be consistent. So it's OK to leave as-is.