feat: Add a way to convert an entire playlist

Closes #194

classes/ConvertedPlaylistArchiveStream.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * ConvertedPlaylistArchiveStream class.
+ */
+
+namespace Alltube;
+
+use Slim\Http\Stream;
+
+class ConvertedPlaylistArchiveStream extends PlaylistArchiveStream
+{
+    /**
+     * Start streaming a new video.
+     *
+     * @param Video $video Video to stream
+     *
+     * @return void
+     */
+    protected function startVideoStream(Video $video)
+    {
+        $this->curVideoStream = new Stream($video->getAudioStream());
+
+        $this->init_file_stream_transfer(
+            $video->getFileNameWithExtension('mp3'),
+            // The ZIP format does not care about the file size.
+            0
+        );
+    }
+}