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/hash/mod.rs

@@ -0,0 +1,8 @@
+mod command;
+mod generic_digest;
+mod md5;
+mod sha256;
+
+pub use self::command::Hash;
+pub use self::md5::HashMd5;
+pub use self::sha256::HashSha256;