Make `fh init` take a language option.

[vic]

It would be nice if fh init could take some CLI option like --lang rust (or any other supported handler) because people will tend to initialize a flake on an empty directory.

For a rust project, I created a directory and had to touch an empty Cargo.toml before running fh init -- and wont even have cargo on my PATH until the flake is ready.

What do you think? I'd love to contribute this.