Change JSON instances for Opt to TemplateHaskell (#4083)

The `Generic` JSON instances for `Text.Pandoc.App.Opt` seem to tickle a
particulary bad quadratic complexity case (Generics complexity is worse
than quadratic with respect to the number of fields in the datatype).
This is with GHC-8.2.1, I didn't test it using 8.0 but I assume it is
similar.

Using `Generic`, compilation of the `Text.Pandoc.App` module takes
minutes and often gets killed due to out of memory on slower machines
with "only" 8GB of accessible memory.  This is particularly annoying to
me since it means I cannot build pandoc on Travis.

TemplateHaskell is a little uglier, but the module seems to compile
within a few seconds, and compilation doesn't take more than 1GB of
memory.

Should I also change the other JSON instances throughout the codebase
for consistency?

src/Text/Pandoc/App.hs

@@ -1,6 +1,7 @@
 {-# LANGUAGE CPP                 #-}
 {-# LANGUAGE DeriveGeneric       #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TemplateHaskell     #-}
 {-# LANGUAGE TupleSections       #-}
 {-
 Copyright (C) 2006-2017 John MacFarlane <jgm@berkeley.edu>
@@ -44,8 +45,8 @@ import qualified Control.Exception as E
 import Control.Monad
 import Control.Monad.Except (catchError, throwError)
 import Control.Monad.Trans
-import Data.Aeson (FromJSON (..), ToJSON (..), defaultOptions, eitherDecode',
-                   encode, genericToEncoding)
+import Data.Aeson (defaultOptions, eitherDecode', encode)
+import Data.Aeson.TH (deriveJSON)
 import qualified Data.ByteString as BS
 import qualified Data.ByteString.Lazy as B
 import Data.Char (toLower, toUpper)
@@ -95,10 +96,6 @@ import System.Posix.Terminal (queryTerminal)
 
 data LineEnding = LF | CRLF | Native deriving (Show, Generic)
 
-instance ToJSON LineEnding where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON LineEnding
-
 parseOptions :: [OptDescr (Opt -> IO Opt)] -> Opt -> IO Opt
 parseOptions options' defaults = do
   rawArgs <- map UTF8.decodeArg <$> getArgs
@@ -646,10 +643,6 @@ data Opt = Opt
     , optStripComments         :: Bool       -- ^ Skip HTML comments
     } deriving (Generic, Show)
 
-instance ToJSON Opt where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON Opt
-
 -- | Defaults for command-line options.
 defaultOpts :: Opt
 defaultOpts = Opt
@@ -1677,3 +1670,6 @@ deprecatedOption o =
     \r -> case r of
        Right () -> return ()
        Left e   -> E.throwIO e
+
+$(deriveJSON defaultOptions ''LineEnding)
+$(deriveJSON defaultOptions ''Opt)