WP-CLI support for authors for a post

[johnbillion]

When updating posts via WP-CLI with wp post update, there should be a flag that can be used to set multiple post authors.

• Should work the same as other WP-CLI flags that accept a list of IDs, most likely a comma separated list
• Introduce this as a new flag, eg --authorship=1,2,3
• The underlying behaviour can probably be enabled by a custom parameter for wp_insert_post(), but needs investigating