Be more accepting for generation options.
Be more accepting for generation options.

Don't require strict casing of the `OutputNaming` and `Visibility` generation
options. Also accept underscore splits on `OutputNaming`.

Stop the two Enums from being string based, since they don't rely on that
anymore.

thomasvl/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf