Knack

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* Adds ability to declare that command groups, commands, and arguments are in a preview status and therefore might change or be removed. This is done by passing the kwarg `is_preview=True`.
* Adds a generic ``StatusTag`` class to ``knack.util`` that allows you to create your own colorized tags like ``[Preview]`` and ``[Deprecated]``.
* When an incorrect command name is entered, Knack will now attempt to suggest the closest alternative.

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* Always read from local for configured_default

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* Support local context chained config file

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* Allows the loading of text files using filename syntax.
* Adds the argument kwarg configured_default to support setting argument defaults via the config file's [defaults] section or an environment variable.

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* Removes an incorrect check when adding arguments.

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* Updates usages of yaml.load to use yaml.safe_load.