Add --help and print usage if no args specified

[ian-h-chamberlain]

Hi, hope it's okay I decided to try and add this without opening an issue first. Let me know if you'd like to see any changes.

• Replace panics for invalid args with help message and exit 2
• Add -h / --help args (exit 0 when these are used)
• Cleanup minor clippy lints found along the way

[Meziu]

This is great, thank you for the collaboration. Still, don't think about cargo-3ds too much, since it needs a rewrite. Mainly about #1 and #2

[ian-h-chamberlain]

Sounds good, I mainly wanted this as a way to check cargo-3ds installation for use with cargo-make's test_arg. Presumably all this code would get replaced by a proper CLI library or something at some point.

I definitely plan to focus my contribution efforts more on the other crates 👍