Text.PrettyPrint.Reader

I found this wrapper code around Text.PrettyPrint.HughesPJ to be useful when wishing to do IO operations in the course of pretty-printing. It allows the pretty-printing combinators to be used in much the same way as before, but also allows lifted operations to be called for things such as db lookups.

Although this code seems generally useful to me, I'm not sure whether it belongs in haskell-proper, or in a separate library. Please let me know what you suggest. 

Also, I'm not sure whether the PP type should be exposed as a synonym for ReaderT as it is now, whether ReaderT should be hidden in some way, or just used directly in the interface.