don't add default trackers when { tracker: false }

When the user sets `{ tracker: false }` on the WebTorrent client, don’t
add trackers to the torrent by default.

index.js

@@ -206,6 +206,7 @@ WebTorrent.prototype.seed = function (input, opts, onseed) {
   // When seeding from filesystem, initialize store from that path (avoids a copy)
   if (typeof input === 'string') opts.path = path.dirname(input)
   if (!opts.createdBy) opts.createdBy = 'WebTorrent/' + VERSION_STR
+  if (!self.tracker) opts.announce = []
 
   var streams
   var torrent = self.add(undefined, opts, function (torrent) {

lib/torrent.js

@@ -234,12 +234,12 @@ Torrent.prototype._processParsedTorrent = function (parsedTorrent) {
     parsedTorrent.announce = parsedTorrent.announce.concat(this.announce)
   }
 
-  if (global.WEBTORRENT_ANNOUNCE) {
+  if (this.client.tracker && global.WEBTORRENT_ANNOUNCE) {
     // So `webtorrent-hybrid` can force specific trackers to be used
     parsedTorrent.announce = parsedTorrent.announce.concat(global.WEBTORRENT_ANNOUNCE)
   }
 
-  if (parsedTorrent.announce.length === 0) {
+  if (this.client.tracker && parsedTorrent.announce.length === 0) {
     // When no trackers specified, use some reasonable defaults
     parsedTorrent.announce = createTorrent.announceList.map(function (list) {
       return list[0]