add webtorrent

WebTorrent is a streaming torrent client for node.js and the browser. YEP, THAT'S RIGHT. THE BROWSER. It's written completely in JavaScript – the language of the web – so the same code works in both runtimes.

In the browser, WebTorrent uses WebRTC (data channels) for peer-to-peer transport. It can be used without browser plugins, extensions, or installations. It's Just JavaScript™.

More info: https://github.com/feross/webtorrent
Relevant issue: https://github.com/feross/webtorrent/issues/219

Thanks to @jimaek for recommending jsDelivr.