Implement `wp menu` command

A fresher look at #689

* [*] `wp menu list`
* [*] `wp menu create`
* [ ] `wp menu add-location`
* [ ] `wp menu remove-location`
* [*] `wp menu delete`
* [ ] `wp menu item list`
* [ ] `wp menu item add-<type>`
* [ ] `wp menu item delete`