External trackers

Based on #46 from @arestov
Additional trackers can now be provided in options which allows to speed up peer discovery and enables all kinds of different scenarios, such as using magnet links without relying on DHT.

Added a bunch of tests to verify. ~~There's a bug in bittorent-tracker (feross/bittorrent-tracker#19) which breaks some of the tests, so there's a temporary workaround in place.~~