Unique short name

[GregorySchwartz]

The auto generation of the short names is fantastic, but it would be nice to have it automatically detect used names and choose the next best thing. So if two options are "--cheese" and "--cherries", it would be "-c" and "-C" respectively, or if that is taken, "-d", etc.

[Gabriella439]

One of the goals of the optparse-generic library is that the user should be able to easily predict what command line options the Haskell type will correspond to. Implementing this makes it harder to reason about that translation process

I also think at this level of customization I would recommend using the underlying optparse-applicative library