Add option to keep only the meshes

[Breush]

Some files like FBX support lights and cameras, but these are not always useful.

Assimp supports an importer post-process called aiProcess_RemoveComponent and configured with:

importer.SetPropertyInteger(AI_CONFIG_PP_RVC_FLAGS, aiComponent_CAMERAS | aiComponent_LIGHTS);

We might want yoga to remove everything but meshes/materials by default, and provide a --no-cleanup option in the CLI and such.