Show waveform for audio files

[feross]

Use https://github.com/katspaugh/wavesurfer.js
Demo: http://www.wavesurfer.fm/

@feross here's a quick preview...one question though

1. Right now I'm using this module to detect the file's MIME type and overriding all audio files to use this widget instead. Is there a better solution you can think of?

I'll be adding controls next.

[feross]

I think the right place for this change is in webtorrent in lib/append-to.js so that all users of WebTorrent get this feature!