Setup.hs helper for doctests running

 Currently (beginning of 2017), there isn't a `cabal doctest`
 command. Yet to properly work, doctest needs plenty of configuration.
 This library provides the common bits for writing custom Setup.hs
 until that is resolved.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.