Allow sorting torrents by (first) tracker URL

This patch allows sorting torrents by their "main" tracker's host
name. The first tracker is considerd the main tracker for this purpose.
I find this useful for keeping track of which torrents belong to which
private tracker.

The special sorting key `_tracker` is introduced here. By prefixing
the name with '_', conflicts with keys from the torrent attributes should
be avoided (though this shouldn't really matter, seeing how a special
case handles `_tracker` anyway).