Implement `wp menu` command

A fresher look at #689

* [x] `wp menu list`
* [x] `wp menu create`
* [x] `wp menu theme-locations`
* [x] `wp menu assign-location`
* [x] `wp menu remove-location`
* [x] `wp menu delete`
* [x] `wp menu item list`
* [x] `wp menu item add-<type>`
* [x] `wp menu item delete`