Add an 'announce' option to announce to custom trackers …
$ webtorrent seed somefile -a http://localhost:8000/announce -a udp://localhost:8000 -q magnet:?xt=urn:btih:066ed351804835cf0fec21e3b352f58e270a26d5&dn=somefile&tr=http%3A%2F%2Flocalhost%3A8000%2Fannounce&tr=udp%3A%2F%2Flocalhost%3A8000