Added WP-CLI support to workspace (#1748)

* Added WP-CLI support
* Updated documentation for WP-CLI

env-example

@@ -104,6 +104,7 @@ WORKSPACE_INSTALL_MSSQL=false
 WORKSPACE_INSTALL_DRUSH=false
 WORKSPACE_DRUSH_VERSION=8.1.17
 WORKSPACE_INSTALL_DRUPAL_CONSOLE=false
+WORKSPACE_INSTALL_WP_CLI=false
 WORKSPACE_INSTALL_AEROSPIKE=false
 WORKSPACE_INSTALL_V8JS=false
 WORKSPACE_INSTALL_LARAVEL_ENVOY=false