Add allNullaryConstructorTagModifier option

[wi11dey]

Often, enums (all nullary contructors) are encoded differently than sum types.

For example, if I have

data State = Launching | Launched | Terminated | MarkedForDeletion

I might want to have these states encoded as LAUNCHING, LAUNCHED, TERMINATED, MARKED_FOR_DELETION, respectively.

However, using the existing constructorTagModifier is not satisfactory for this because I don't necessarily want my sum types, e.g.

data Employee = Manager { subordinates :: [Employee] } | Junior

to be encoded the same way as an enum.

I've added an optional field allNullaryConstructorTagModifier (it's Maybe so that there's no behavior change to any existing usages of Aeson, in case someone is depending on constructorTagModifier also applying to all-nullary datatypes). When it's Nothing, everything is the same as today. When it's non-Nothing it's used instead of constructorTagModifier for all-nullary datatypes, for the purposes of handling enums differently than sum types.

Please let me know if you'd like any changes to the api