Split out shell-piping :: Maybe

I am very interested in the ability mix unix-pipe streams with native Haskell streams. All I really need to do this is [pipes-shell](http://hackage.haskell.org/package/pipes-shell-0.1.3/docs/Pipes-Shell.html), but I was surprised to find that `turtle` doesn't even use that library. It must be that you're accomplishing it some other way. That functionality by itself is quite helpful. Is there a reason you implemented it differently than `pipes-shell` and/or why your version is not its own package?

The issue names turtle's shell-piping functionality and the pipes-shell 0.1.3 package; begin by reading the linked Pipes-Shell documentation and locating turtle's corresponding implementation. Clarify whether the goal is compatibility, an explanation of the differing implementation, or a standalone package; done requires a maintainer decision and agreed scope.

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