Hugo: Check content for dead links during the generation

[digitalcraftsman]

This optional feature could check if a link inside a document is and output a warning.

[bep]

This sounds out of scope. The only way to check dead links is to "try it" -- and that is too slow to be part of Hugo's build process.

[carlmjohnson]

I think we could add a separate subcommand for this, like hugo test. For the site I'm working on, I'd like to have a step that will check for broken links before deploying. I could do this by just building on localhost and running a link checker over that, but ISTM it'd be faster/more efficient if it were built into Hugo.