Flag to add known-good open trackers

https://github.com/webtorrent/create-torrent adds these trackers:

- udp://tracker.openbittorrent.com:80
- udp://tracker.internetwarriors.net:1337
- udp://tracker.leechers-paradise.org:6969
- udp://tracker.coppersurfer.tk:6969
- udp://exodus.desync.com:6969
- wss://tracker.btorrent.xyz
- wss://tracker.openwebtorrent.com
- wss://tracker.fastcast.nz

It might be nice to do this dynamically, using some auto-updated list of known good trackers. This looks like a good list: https://github.com/ngosang/trackerslist