jekyll-relative-links: Detect broken links

Is your feature request related to a problem? Please describe the problem you’re trying to solve.

Suppose I create a link like this:

See [the help page](support/help.md) for more troubleshooting info.

Describe the solution you’d like

If ./support/help.md would be a broken link at runtime, then Jekyll should fail with an error at build time.

Additional context

Currently jekyll-relative-links seems to silently emit a broken hyperlink.