Support passing `--parent-id` to set the parent of a menu item
When updating a menu item, always persist its data

Fixes a bug in core where the data will be lost https://core.trac.wordpress.org/ticket/28138
Persist the appropriate position when updating a menu item

See https://core.trac.wordpress.org/ticket/28140
Fix coding standards error
Merge pull request #1182 from wp-cli/fix-parent-id-1182

wp menu item add-post --parent-id=ID does not set menu item parent id