Add cargo feature to use alpha-3 for ser/de ?

[mwilliammyers]

Hello!

Since the enum variants use alpha-3, I think it might make sense to use them in the ser/de implementation (by default), but failing that, at least have a feature to switch to alpha-3 for ser/de impls. What do you think?

I would be happy to submit a PR for it if you think it would be a good idea.