Should not be necessary to pass empty `messages`.

[alat-rights]

Mentioned in #26

The current way to create a messages list in alana is this:

messages = []
alana.gen(user="...", messages=messages)

I think it's a considerable improvement over the status quo with the Anthropic API.

However, I think it'd be better if the user could just be returned a messages list. Should this be a modification to current API behavior, or behavior given flag, or something else?