CLI: enable comma delimited list of post ids to classifai

[adamsilverstein]

Description of the Change

Explode the comma delimited list of post ids passed to the command.
Adjust the command docs to make it clear what should be passed.

Verification Process

type a comma delimited list of posts to classifai, eg:

wp classifai post 3602,3604,3606

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

[jeffpaul]

@adamsilverstein is there a parallel update to the readme that should explicitly note how this works / provide an example?

[adamsilverstein]

@jeffpaul the PR updates the docs you get when typing the CLI command, not sure if this is documented elsewhere, I'll look

[jeffpaul]

@adamsilverstein can you please update the readme as well to denote the update to the command?

[adamsilverstein]

Added docs to the readme (as well as some editor induced whitespace cleanup) - see https://github.com/10up/classifai/pull/55/files?w=1 for the ignore whitespace changes view.