Backend-ai-client

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**NEW**

- Now the CLI supports "vfolder" subcommands.

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**CHANGES**

- Now it uses "api.backend.ai" as the default endpoint.

- It also searches `BACKEND_`-prefixed environment variables first and then
falls back to `SORNA_`-prefixed environment variables as legacy.

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**CHANGE**

- Remove `simplejson` from our dependencies.

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**NEW**

- Add "-s" / "--stats" option to the CLI "run" command.
When specified, the CLI shows resource usage statistics after session termination.

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**NEW**

- Now you can run the CLI commands using "backend.ai"
instead of "python -m ai.backend.client.cli"

- Add a few new CLI commands: config, help, ps

- Running "backend.ai" without any args shows the help message
instead of an error.

**FIX**

- Fix colored terminal output in *NIX (12)

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**FIX**

- Make the colored terminal output working on Windows (12)