Add local link checker

[dankohn]

It's easy to create broken internal links. Please add a build step that checks that all internal links work.

Probably best option:
https://github.com/wjdp/htmltest/wiki/Using-With-Hugo

Other options:
https://github.com/stevenvachon/broken-link-checker
redhat-cop/uncontained.io#103

https://github.com/raviqqe/muffet

You might also check external links, but it would probably be best for speed if external links weren't checked on every build, as one temporarily down external site could break the build.