Added threefish

[newpavlov]

Code based on cbarcenas's implementation with his consent.

[Slals]

Out of curiosity, it seems you forced-push treefish but nowhere to be found in master @newpavlov, any reason?

[Nakilon]

How do I use it? Never used Rust.
I've installed cargo, cargo show and it pointed at this repo. Now what? I would like to use it in a terminal to encrypt files with threefish-256.