Add basic autocompletion

[briankassouf]

No description provided.

[brianshumate]

👋 I accidentally duplicated some of this effort today, but here's the initial docs section I'd written for docs/commands/index in hopes that they provide a good starting point:

Autocompletion

The vault command features opt-in subcommand autocompletion that you can
enable for your shell with vault -autocomplete-install. After doing so,
you can invoke a new shell and use the feature.

For example, assume a tab is typed at the end of each prompt line:

$ vault au
audit-disable  audit-enable  audit-list  auth  auth-disable  auth-enable

$ vault s
seal  server  ssh  status  step-down

[jefferai]

@brianshumate Probably should be vault -autocomplete-install :-D

[brianshumate]

Corrected! 😉

[briankassouf]

@brianshumate Thanks! Added that to the docs section