Added upload validate and preview cli command subcommand.

packages/python/docs/cli.md

packages/python/src/snip/snippets/__main__.py

packages/python/src/snip/snippets/base.py

packages/python/src/snip/snippets/image.py

packages/python/src/snip/snippets/text.py

packages/python/src/snip/token/storage/__init__.py

packages/python/tests/test_api.py

packages/python/tests/test_cli.py

packages/python/tests/test_storage.py