Change magnetURI to torrentId

Change magnetURI to torrentId to avoid confusing, because client.add is not limited only to magnet URIs and there are many ways to define torrentId, e.g. HTTP(S) URL to a *.torrent file.

README.md

@@ -145,9 +145,9 @@ standards (no plugins, just HTML5 and WebRTC)! It's easy to get started!
 var WebTorrent = require('webtorrent')
 
 var client = new WebTorrent()
-var magnetURI = '...'
+var torrentId = '...'
 
-client.add(magnetURI, function (torrent) {
+client.add(torrentId, function (torrent) {
   // Got torrent metadata!
   console.log('Client is downloading:', torrent.infoHash)