Port hash, hash md5 and hash sha256 commands (#464)

`hash` by itself is only printing the help message.

The other two are simply using the same generic implementation.

crates/nu-command/src/lib.rs

@@ -9,6 +9,7 @@ mod experimental;
 mod filesystem;
 mod filters;
 mod formats;
+mod hash;
 mod math;
 mod network;
 mod platform;
@@ -29,6 +30,7 @@ pub use experimental::*;
 pub use filesystem::*;
 pub use filters::*;
 pub use formats::*;
+pub use hash::*;
 pub use math::*;
 pub use network::*;
 pub use platform::*;