Separate line number count from code to allow for copying, closes #42

Trying to copy the code would give you a bunch of line numbers that needed to be edited out by hand. The horrors of this experience are now over. A pull request has come to save us. 

Because this pull request is the hero that better_errors deserves, but not the one it needs right now... although it would be helpful to have anyway... And so we'll merge it into master, because it can take it. Because this pull request is not a hero, it's a silent guardian, a watchful protector, a Dark Knight...