RFE: apkg clean

rm -rf is an infamously dangerous command, let apkg handle cleaning without the need to specify paths (room for error).

• apkg clean to rm -rf pkg/ (clean output dir)
• apkg clean --cache to only rm pkg/.cache.json (clean cache)
• apkg clean --nuke to hard reset repo and remove extra files (requires VCS)

git reset --hard
git clean -fxd