Add a --version flag that prints the crate version and exits #68

Add pub version: bool to Options in src/lib.rs, set by --version in parse_args. In src/main.rs, when set, print env!("CARGO_PKG_VERSION") (e.g. 0.1.0) and exit with status 0 without printing a greeting. Add a unit test for parsing the flag, and an integration test in tests/cli.rs that runs the binary (use env!("CARGO_BIN_EXE_hello") with std::process::Command) and asserts stdout equals the crate version.

Definition of done

• cargo test passes.
• cargo run -- --version passes.