Add support for -h/--help arg

[HaoboGu]

When use flip-link directly from command line, now flip-link returns an error code 1:

$ flip-link
lld is a generic driver.
Invoke ld.lld (Unix), ld64.lld (macOS), lld-link (Windows), wasm-ld (WebAssembly) instead

flip-link: the native linker failed to link the program normally; please check your project configuration and linker scripts

It's inconvenient when we want to just test whether flip-link exists. In my case, I use cargo-make to automate the build workflow, it checks flip-link command's return code to determine whether to install the crate, so it will re-install flip-link everytime even though flip-link has been installed.

This PR adds -h/--help argument. When user executes flip-link -h or flip-link --help or flip-link in command line, flip-link now returns:

$ flip-link
flip-link: adds zero-cost stack overflow protection to your embedded programs

You should not use flip-link directly from command line, use flip-link as your default linker instead. 
For detailed usage, check https://github.com/knurling-rs/flip-link

[Urhengulas]

lgtm

[Urhengulas]

Oh sorry, I put in a mistake in my recommendation. Can you please fix it, add a CHANGELOG entry and squash all of it into one commit please?

[Urhengulas]

Thank you for the contribution and thank you for bearing with me 😉