0.8.0

• Adds --verify flag, similar to --dry-run but returning 1 if any
  affected files would have changed (@jspaezp, #153, #195).
• Adds script to apply nbstripout retroactively using git-filter-repo to
  README (@LunarLanding @tokleine, #194 #197).
• Documents nbstripout on all files in the current directory and
  subdirectories recursively (#127).
• Accepts notebooks without nbformat version specified.
• Improves test coverage.