Sort issues

Now external issues come out in random order:

```bash
# 0
_site/docs/history/index.html: External link https://github.com/bahuvrihi failed: 404 No error
# 0
_site/docs/history/index.html: External link https://github.com/gcnovus failed: 404 No error
# 1
_site/docs/deployment-methods/index.html: External link https://github.com/rtomakyo/shotgun/ failed: 404 No error
# 2
_site/docs/extras/index.html: External link http://kramdown.rubyforge.org/ failed: got a time out
# 1
_site/docs/deployment-methods/index.html: External link http://net-ssh.rubyforge.org/ failed: got a time out
# 3
_site/docs/plugins/index.html: External link http://ultraviolet.rubyforge.org/ failed: got a time out
# 2
_site/docs/extras/index.html: External link http://kramdown.rubyforge.org/options.html failed: got a time out
# 3
_site/docs/plugins/index.html: External link http://saswatpadhi.github.io/ failed: 404 No error
```

Sorted issues would support the cleaning workflow. It’s handy.