Surface dead links in results output

[victoriadrake]

• Show number of dead links found
• Make it easier to see dead link URLs when link check finishes by
  grouping them at end of output

[victoriadrake]

Hello @tcort! Thank you for writing such a tidy link checker, it's been working well for me. I thought I'd share some additions to the program that have helped me spot dead links when dealing with a lot of output. My apologies if the implementation is less than elegant; my JS has gotten a touch rusty. ^^;