extract assoc_array_to_table() utility.

Also:
- unset the 'format_content' field; it can be just as long as
'post_content'.
- unset the 'filter' field; it's a runtime value, not persistent data
implement `wp user get`
behat: align table bars
WP_User->to_array() doesn't exist in WP 3.4
Merge pull request #595 from wp-cli/user-get

Implement `wp user get`