Possible cache support

Use cache for sending previously downloaded videos

src/index.ts

@@ -124,12 +124,13 @@ class YouTubeVideoClipEventEmitter extends EventEmitter implements ClipEventEmit
       ffmpegProcessAudioStreamArgs.push(clipDuration);
     }
 
+    const cacheFilename = 'output.webm'; // todo label this on the cache
     const ffmpegProcessArgs = [
       ...ffmpegProcessVideoStreamArgs,
       ...ffmpegProcessAudioStreamArgs,
       '-map', '0:v',
       '-map', '1:a',
-      'output.webm',
+      cacheFilename,
     ];
     this.emit('process', {
       type: 'postprocess',
@@ -142,8 +143,8 @@ class YouTubeVideoClipEventEmitter extends EventEmitter implements ClipEventEmit
       this.emit('end');
       return;
     }
-    const output = readFileSync('output.webm');
-    unlinkSync('output.webm');
+    const output = readFileSync(cacheFilename);
+    unlinkSync(cacheFilename);
     this.emit('process', {
       type: 'postprocess',
       phase: 'success',