fix: handle stdio properly

examples/hello-world.php

@@ -0,0 +1,24 @@
+<?php
+
+use Nih\CommandBuilder\Command;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+$output = (new Command('echo'))
+    ->arg('Hello, World!')
+    ->output();
+
+var_dump($output);
+
+// object(Nih\CommandBuilder\Output)#9 (3) {
+//   ["stdout"]=>
+//   string(14) "Hello, World!
+// "
+//   ["stderr"]=>
+//   string(0) ""
+//   ["code"]=>
+//   object(Nih\CommandBuilder\ExitStatus)#8 (1) {
+//     ["code"]=>
+//     int(0)
+//   }
+// }