Add option for stdio encoding, set stdin encoding

Language/Haskell/GhcMod/Types.hs

@@ -105,6 +105,7 @@ data Options = Options {
     -- | GHC command line options set on the @ghc-mod@ command line
   , optGhcUserOptions :: [GHCOption]
   , optFileMappings   :: [(FilePath, Maybe FilePath)]
+  , optEncoding       :: String
   } deriving (Show)
 
 -- | A default 'Options'.
@@ -124,6 +125,7 @@ defaultOptions = Options {
     }
   , optGhcUserOptions = []
   , optFileMappings   = []
+  , optEncoding       = "UTF-8"
   }
 
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