Ability to tag commands as "Experimental" or "Deprecated"

[mcwumbly]

In issue #5 / pr #7, the ability to tag flags as deprecated or experimental was added. It'd be nice to do the same for commands.

Per @genevieve:

We could add two fields to this struct that say Deprecated bool and Experimental bool and then alter their description/short-description.

[cf-gitbot]

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