Reset block args only when there's no opened blocks

See #32

src/Blocks/Block.php

@@ -67,7 +67,7 @@ public function close()
         }
         $args = $this->args;
         $this->opened--;
-        $this->args = [];
+        $this->opened === 0 and $this->args = [];
         array_unshift($args, ob_get_clean());
         $block = call_user_func_array($this->callback, $args);