[Protocol] Add PROPRIETARY to protocol enumeration

[CosminPerRam]

When browsing source in node-gamedig, I don't really like that in the protocols folder there also are located the proprietary ones, it doesn't feel right to me, so here I added an additional wrapping layer of the Protocol enum (in generics), this is strictly done to improve readability (also a way to know that the protocol of game X is proprietary).

[Douile]

I'm being a bit nitpicky. But I think it would be better if Protocol::PROPRIETARY and CustomProtocol had a consistent name if the only use of CustomProtocol will be there.