Add support for WP-CLI

NAME
  wp polylang

DESCRIPTION
  Implements polylang command, to interact with the Polylang plug-in.

SYNOPSIS
  wp polylang <command>

SUBCOMMANDS
  get            Gets a post or a term in the specified language
  home           Gets the site homepage URL in the specified language
  language       Adds, gets information or remove a language
  languages      Prints the languages currently available