[Shock] - Create plugin that checks for broken links

[matthew-carroll]

It's easy to type links with typos when authoring articles. Create a plugin that checks page content, before it's written to the final file, which validates every URL that it finds in the HTML content.

The plugin should support a quiet mode, which says nothing, a warning mode, which outputs warning logs during build, and an error mode, which stops the build upon finding any broken links.

When in error mode, the plugin should check all URLs before stopping the build. This way, the user doesn't have to play whack-a-mole, fixing one link per build.