Custom fork enums for non Ethereum use cases

[karalabe]

Currently the library ships the Ethereum hard forks in an enum, allowing it to be used across the lib, as well as having built in support in the code generator.

Whilst users of the library can use their own package for the fork enum (i.e. it's just an int), the code generator does not support mapping custom fork names to custom external library package enums. This should not be too hard to add, but it's also not something immediately on my plate.

I.e. this issue is just a braindump to signal this missing feature.

If anyone wants to take a stab at it, please make sure you discuss it here first as the design and integration needs to fit with the rest of the code without breaking too many things.