Also extended a bit the options for batch exporting (collections give
more ways to organize your scene).

And added non-batch option to only export objects from active Collection.

'partial' support because until we can force generate depsgraph for
inactive view layers, we cannot support exporting instances (dupli
objects etc.) for batches.

TODO: move the batch logic to IO helpers, that could be another
decorator, to easily add the same feature to all exporters able to
export several objects...