Summary: Refactor ply_io to make it easier to add new features. Mostly taken from the starting code I attached to https://github.com/facebookresearch/pytorch3d/issues/904.

Reviewed By: patricklabatut

Differential Revision: D34375978

fbshipit-source-id: ec017d31f07c6f71ba6d97a0623bb10be1e81212