Add an empty help message to the nginx-vhosts plugin.

Not doing anything just destroys the help message coming from previous
plugins. Run cat to forward stdin into stdout.

plugins/nginx-vhosts/commands

@@ -1,2 +1,9 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+case "$1" in
+  help)
+    cat
+    ;;
+esac
+