btrtrc -- better BitTorrent client

This is a fork of https://github.com/anacrolix/torrent BitTorrent
library with own cmd/btrtrc client implementation. Comparing to
cmd/torrent it has much less configuration options, mainly hardcoding
the most of them. But what advantages does it have?

* Optimized file-based storage:
  * linearized I/O operations prevent creation of huge quantity of threads
  * cached file descriptors save a lot of syscalls
* Shortened long filenames in file-based storage
* Ability to specify both IPv4 and IPv6 addresses to announce
* Ability to specify DHT bootstrap nodes
* Dynamic addition and removing of the torrents
* Much richer and nicer coloured status output
* Ability to calculate seed ratio, by remembering outgoing traffic amount
* Simpler piece completion database per each torrent in separate files
* Ability to sequentially create/verify torrent's piece completion database
* Optional commit disables any WebTorrent/WebRTC/WebSocket usage,
  because those technologies suck

Look at USAGE file further.

btrtrc is free software: see the file LICENSE in the root of the
repository for copying conditions.