download: add an option to save a single subset

[IRDonch]

Summary

This option is similar to the --subset option in the validate command. It was requested by the DL Workbench team.

Note that it only affects saving. TFDS will still download all subsets, even if only one is requested.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[IRDonch]

CC @vgoluben