Make postprocessing configurable

[georgefst]

Ended up writing a fix for #39, to help with a script I was writing this afternoon.

Not quite sure about the details of the API yet.

Tests expected to fail right now, though fixes should be trivial. And new tests will be added anyway.

[georgefst]

This opens up a lot of flexibility. But it feels slightly ugly, since we will never actually parse a CustomExpr. We could formalise that fact with GADTs and DataKinds, but it's possibly unwarranted complexity.

[cdepillabout]

This looks pretty good to me!

[georgefst]

I'm spitballing here, and I'm not sure where it falls on a spectrum anywhere from making this redundant to being completely orthogonal, but anyway, if I ever return to this, I think we should look in to integrating syb somehow to make it easy to override show output for certain types:

import Data.Data
import Data.Generics.Aliases
import Text.Pretty.Simple

gshowWith :: (Data a, Typeable b) => (b -> String) -> a -> String
gshowWith f x = gshowsWith f x ""
gshowsWith :: (Data a, Typeable b) => (b -> String) -> a -> ShowS
gshowsWith f =
    ( \t ->
        showChar '('
            . (showString . showConstr . toConstr $ t)
            . (foldr (.) id . gmapQ ((showChar ' ' .) . gshowsWith f) $ t)
            . showChar ')'
    )
        `extQ` (shows :: String -> ShowS)
        -- only here do we modify the definition of `gshows`
        `extQ` ((++) . f)

data Example = Example Int String Word | Rec () Example deriving (Data, Typeable)
example :: Example
example = Rec () $ Example 42 "Hello" 15

main :: IO ()
main =
    pPrintString $
        gshowWith
            (\(i :: Int) -> "<int: " ++ show i ++ ">")
            example

( Rec ( () )
    ( Example <int: 42> "Hello" ( 15 ) )
)