Implement a solution for additional license exceptions.

[dsully]

Implement a solution for additional license exceptions configuration as described in #541

Happy to entertain other solutions or approaches.

Minor typo fixed as well.

[Jake-Shadle]

Apologies for taking so long to look at this!

I think the code in this would work to solve the immediate use case you have, but I'm worried if I accept it it would make adding a more general layered configuration system, similar to what cargo has, more difficult in the future. But I suppose I can accept it for now, just with the understanding if that layered configuration system is done in the future you will need to migrate to the new one, which is probably fine since since I don't think many people will use it.

[dsully]

Thanks - I agree that the layered approach is better, but the way cargo-deny is currently implemented didn't lend itself to doing that (easily at least), and I needed a targeted solution.

Thanks for accepting the PR, and let me know if I can help on the future change.