Allow skipping to a specific test number or shrink state

[ChickenProp]

I just opened a PR for this feature on hedgehog: hedgehogqa/haskell-hedgehog#454

I don't know if it's going to be accepted. But I bring it up here because QuickCheck may also want such a feature. (I might be up for implementing it myself, but not in the immediate future at least.) And if both packages are going to have it, then sharing discussion might be valuable? Like there might be details of how it would ideally behave, or about how it might integrate with hspec, where input from quickcheck could be useful.