Use letters instead of numbers as the suffix for arg variables

[MartinSStewart]

I'm guessing you don't have any preference between using arg0, arg1, arg2 ... vs using argA, argB, argC ... ? If that's true then I'd like for elm-derive to use letters instead. The reason being that I have an elm-review rule that prevents the user from using "stale references". i.e. if there's a model2, you can't use model. I find this useful for catching bugs, but it's a bit inconvenient when elm-derive generates arg0 and arg1 and uses both at the same time which my rule flags.