Add support for advanced output

[clockworksoul]

Currently, command output is limited to plain text, as if it was any command line tool. This is adequate for simple use cases, but doesn't allow for the addition of non-response data or metadata.

Commands optionally using "advanced output" would be able to overcome this limitation by returning rich-JSON-formatted responses that can contain directives including (but not limited to):

• Structured response data, which can be processed by templates
• Directives to the chat provider, such as adding reactions to chats

Response text would also be optional, and omitting it would mean that no output is generated in the chat provider.

This is a significant feature, and a will require a well-considered proposal.