Feature request: support autocompletion in plugins

[kitd]

Hello. We (IBM Event Streams) are writing a cloudctl plugin.

We would like to support dynamic autocompletion of flag values in the plugin but the --generate-bash-completion flag never makes it through to our plugin to allow this to happen.

There doesn't appear to be an option to define global plugin flags, which would have been an acceptable alternative.

So we have had to inject an hidden --autocomplete flag to each command, which is a pain to maintain and test.

Could we request either to have the --generate-bash-completion flag passed through to the plugin, or allow global flags to be defined for a plugin?

[weieigao]

@kitd see #177.