feat: add support for --global flag when calling dokku domains

docs/configuration/domains.md

@@ -3,7 +3,7 @@
 > New as of 0.3.10
 
 ```
-domains [<app>]                                # List domains
+domains [<app>|--global]                       # List domains
 domains:add <app> <domain> [<domain> ...]      # Add domains to app
 domains:add-global <domain> [<domain> ...]     # Add global domain names
 domains:clear <app>                            # Clear all domains for app

plugins/domains/subcommands/default

@@ -12,6 +12,11 @@ domains_main_cmd() {
     dokku_log_info2_quiet "Global Domain Name"
     get_global_vhosts
   fi
+
+  if [[ "$APP" == "--global" ]]; then
+    return
+  fi
+
   if [[ -n "$APP" ]]; then
     verify_app_name "$APP"
     if [[ -f "$DOKKU_ROOT/$APP/VHOST" ]]; then