Rather than having Pandoc generate the id, which has the potential to change
over time and cause 404s, let's just generate the slug from the filename so
that the ids will never change.  This also solves the awkward question of
what the filename should be, since it was previously something arbitrary.

This mass rename was accomplished via this simple shell script:

  for p in *.meta; do
    slug=$( recsel -P slug "$p" | xargs basename )
    mv -v "${p/.meta/.md}" "${p:0:10}-$slug.md"
  done

with minor manual tweaks where I saw fit.  Of course, now I have some pretty
long filenames, which is undesirable.

The next step is to compare it with the slugs currently on mikegerwitz.com
and make them match.  That's the next commit, and should be pretty simple.