CLI command to migrate legacy WP users

[mattheu]

If you enable Authorship on an existing site, author archives break because authorship authors are not set.

This adds a CLI command to run through all posts that have no authorship author set, and sets adds the post_author to authorship.

Optionally, you can pass --override-authorship to remove all authorship users and replace with WordPress authors.

Fixes #78.

[svandragt]

I don't think the optional parameter is clear at all, but it's a great option to have. How about --set-wordpress-authors to be descriptive of what the end result is?

[johnbillion]

This looks good, just needs:

1. A fix for the dry-run flag
2. Docs added to the readme (probably above the "PublishPress Authors Migration" section)

[mattheu]

I don't think the optional parameter is clear at all, but it's a great option to have. How about --set-wordpress-authors to be descriptive of what the end result is?

@svandragt I copied this command from the Publish Press Authors migration command, which does the exact same thing. So for consistency, I'd like to keep it the same.

[mattheu]

Feedback addressed @johnbillion