Requires silencing an HTML-only error that isn't actually an error, but
a hint that doesn't apply to polygot markup (warns against trailing
slashes which aren't a problem if you always use quotes).
- Avoid relative paths, so I can run them from any working directory.
- Make a dedicated curl-wrapping shell script instead of re-defining the
same alias everywhere.
- Support extended offline periods: allow get-webmentions.sh to fall
back to the cached copy of my webmentions for up to a day, and don't
accidentally destroy it; keep changes atomic.
- Verify that the fetched webmentions are legit before replacing the
cached ones.
- Make shellcheck happy about quoting in vnu.sh by passing the list of
files with xargs instead of a shell variable.
