Add META.json for pgxn.org and script to bundle archives

Please scrutinize META.json thoroughly.

If all looks good, I think this is now ready for PGXN. You can tag 1.0.0 with:
```
git tag -a v1.0.0 -m 'Tag v1.0.0'
```

I will then use archive.sh to generate a PGXN distribution and upload it.