Stdin Support

Ideally, I'd like to be able to use ffmpeg.wasm with the MediaRecorder API, which supplies "chunks" of video. Currently, I use this API in conjunction with ffmpeg via a node `child_process` and using the stdin pipe to "feed" data to ffmpeg while it's running.

I'd love to be able to do something like the following:

```
let instance = ffmpeg.start('-i - -f hls -hls_time 2 -hls_playlist_type event stream.m3u8');
instance.stdin.write(new Uint8Array(arrayBuffer));
```

Currently doing something similar in a node environment, but would love to give WASM a try instead of needing an Electron app.