Add PLY Exporter

[gkjohnson]

Hey! I had to write a PLY exporter, so I figured I'd contribute it back to the THREE exporter examples folder. It outputs ascii PLY-formatted files that can then be re-imported with the existing PLY-Loader. The dev repo is here: https://github.com/gkjohnson/ply-exporter-js.

Limitations

• Can only export ascii-formatted data (no binary format support).
• No ability to export custom attributes.
• Synchronous.
• Missing geometry attributes are not automatically excluded.

[mrdoob]

Very nice!

[mrdoob]

Thanks!