Merge other driver implementation into this crate

[Wassasin]

I created an implementation of this crate earlier @ https://github.com/Wassasin/pcal6416 .

Would you be open to having the work in that crate merged into this repository? I have been using that driver successfully in two projects already, but did not take the effort of actually pushing it to crates.io.

Let me know and I'll make a PR.

[jeffglaum]

@Wassasin , would love it if you're willing to merge it into this repository. Thanks!