Replace github.com/bep/goat with this repo in Hugo

[bep]

Hey,

Happy to see my changes merged back to its original. I was a little impatient when adding some diagram support to Hugo, and looking at the activity in this repo, I assumed that forking this repo would be the fastest way to getting something out the door.

But I maintain way too many repos as it is already, so I would be happy to retire my fork if you're open to look at future pull requests with bug fixes and maybe some improvements. It would be to have one canonical version of this.

[bep]

Note that I would really appreciate if the scope of Goat could be as simple and narrow it is now (ascii => svg). I understand the need for the cmd package, but any PNG option would, in my head, not belong in this repo.