add algorithm: bittorrent v2 merkle root hash

[milahu]

ed2k hash is already supported, so adding this seems legit

implementations of the bittorrent v2 merkle root hash algorithm:

• https://github.com/arvidn/libtorrent - C++
  • https://github.com/arvidn/libtorrent/blob/RC_2_0/src/merkle.cpp
  • https://github.com/arvidn/libtorrent/blob/RC_2_0/src/merkle_tree.cpp
• https://github.com/milahu/bittorrent-v2-merkle-root-hash-py - python
  • more comments in https://github.com/milahu/bittorrent-v2-merkle-root-hash-py/blob/2148ef1e822ea71182eb9f2ffabf7f1347dd5dfe/bt2_root_hash.py#L267

this hash is used to share identical files between multiple torrents

effectively, it splits the file into 16*1024 byte blocks, adds zero blocks to get a power-of-two number of blocks, and builds a binary tree of hashes over these blocks