add tracker merging function

[ghost]

make it so that when an existing torrent is added to picotorrent, the trackers from both torrent files are merged.

example: you have a movie with the nyaa tracker. if you go on another site (lets say bitsearch), which has the same movie but with its own tracker, and you add that file from the 2nd site to picotorrent, the new tracker(s) gets added to the previous one(s), so that, in this case, the torrent will now have the nyaa tracker and the bitsearch one. Instead of saying "torrent already exists, will not add it", it would say "torrent already exists, trackers will be merged".