Blender Git Loki

IO: Allow exporting a subset of the writers

This is in order to prepare for compatibility with the Alembic exporter.
That exporter is capable of writing object transforms and object data at
different (sub)frames.

The rename from `created_writers` to `used_writers` is necessary, as not
all created writers will be actually used in each iteration.

The Universal Scene Description (USD) exporter does not make use of
this.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D7670