Stop exporting writeCustom from module T.P.Writers [API change]

This ensures that all writer exported in T.P.Writers are parameterized
and work with any `PandocMonad` type. This is consistent with
T.P.Readers, as `readCustom` is not exported from that module either.

src/Text/Pandoc/App/OutputSettings.hs

@@ -39,6 +39,7 @@ import Text.Pandoc.App.FormatHeuristics (formatFromFilePaths)
 import Text.Pandoc.App.Opt (Opt (..))
 import Text.Pandoc.App.CommandLineOptions (engines, lookupHighlightStyle,
                                           setVariable)
+import Text.Pandoc.Writers.Custom (writeCustom)
 import qualified Text.Pandoc.UTF8 as UTF8
 
 readUtf8File :: PandocMonad m => FilePath -> m T.Text

src/Text/Pandoc/Writers.hs

@@ -25,7 +25,6 @@ module Text.Pandoc.Writers
     , writeBibLaTeX
     , writeCommonMark
     , writeConTeXt
-    , writeCustom
     , writeCslJson
     , writeDZSlides
     , writeDocbook4
@@ -93,7 +92,6 @@ import Text.Pandoc.Writers.BibTeX
 import Text.Pandoc.Writers.CommonMark
 import Text.Pandoc.Writers.ConTeXt
 import Text.Pandoc.Writers.CslJson
-import Text.Pandoc.Writers.Custom
 import Text.Pandoc.Writers.Docbook
 import Text.Pandoc.Writers.Docx
 import Text.Pandoc.Writers.DokuWiki