add an option to show the version

[KeenS]

This is useful when you want to check the version and install the latest binary if current is old.

$ cargo run -- --help
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/xremap --help`
Usage: target/debug/xremap CONFIG [options]

Options:
        --device NAME   Include a device name or path
        --ignore NAME   Exclude a device name or path
        --watch         Add new devices automatically
    -h, --help          print this help menu
        --version       show version

$ cargo run -- --version
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/xremap --version`
xremap version 0.1.7

For more detail of CARGO_PKG_VERSION, see https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates

[k0kubun]

Thank you!