Add subcommand parsing capability

Mistral client is currently unable to parse the sub-commands.
For Example:
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if the user asks for help on a particular use case, say, workflow-create,

$ mistral workflow-create --help

Actual Output:
All the mistral usecases are being displayed

Expected Output:
Only the help message for workflow-create usecase must be displayed.

To summarize,
"mistral COMMAND --help" must be the same as "mistral help COMMAND" as this is the behavior in all the other projects.