fix: handle stdio properly

examples/plumbing.php

@@ -0,0 +1,17 @@
+<?php
+
+use Nih\CommandBuilder\Command;
+use Nih\CommandBuilder\Stdio;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+$echo = (new Command('echo'))
+    ->arg('Hello, World!')
+    ->stdout(Stdio::piped())
+    ->spawn();
+
+$cat = (new Command('cat'))
+    ->stdin(Stdio::stream($echo->stdout))
+    ->status();
+
+// Prints "Hello, World!\n"