Allow plugging in custom generators for instances

[anka-213]

AFAICT, currently the list of all the different kinds of instances that can be generated is hard-coded into the printer. I would like to be able to plug in some custom code to generate instances for my types, without needing to fork this library.

purescript-bridge/src/Language/PureScript/Bridge/SumType.hs

Line 63 in a65fd63

data Instance = Encode | EncodeJson | Decode | DecodeJson | Generic | Newtype | Eq | Ord deriving (Eq, Show)

purescript-bridge/src/Language/PureScript/Bridge/Printer.hs

Lines 174 to 179 in a65fd63

	instances :: Switches.Settings -> SumType 'PureScript -> [Text]
	instances settings st@(SumType t _ is) = map go is
	where
	go :: Instance -> Text
	go Encode = "instance encode" <> _typeName t <> " :: " <> extras <> "Encode " <> typeInfoToText False t <> " where\n" <>
	" encode = genericEncode $ defaultOptions" <> encodeOpts