Allow subenum visibility to be overridden

[oftheforest]

No description provided.

[oftheforest]

I added an example to the README. It might be nice to also add a usage of vis to the complex example, so that there's an example that shows off all of the features.

I'm not sure that the error messages are the best (then again, are macro errors ever that good?)

syn recommends using syn::Error/compile_error!() over panicking the macro, and I believe syn::Error is the only way to get span highlighting, but that seems like a more invasive change.