Allow binary formats to be written to stdout unless tty output.

Only works on posix.  On Windows, pandoc works as before and
requires an output file parameter for binary formats.

Closes #2677.

pandoc.cabal

@@ -442,6 +442,9 @@ Executable pandoc
   Ghc-Prof-Options: -fprof-auto-exported -rtsopts -with-rtsopts=-K16m
   if os(windows)
     Cpp-options:      -D_WINDOWS
+  else
+    Build-Depends:  unix >= 2.4 && < 2.8
+
   Default-Language: Haskell98
   Other-Extensions: PatternGuards, OverloadedStrings,
                     ScopedTypeVariables, GeneralizedNewtypeDeriving,

pandoc.hs

@@ -74,6 +74,10 @@ import Text.Pandoc.Readers.Txt2Tags (getT2TMeta)
 import Paths_pandoc (getDataDir)
 import Text.Printf (printf)
 import Text.Pandoc.Error
+#ifndef _WINDOWS
+import System.Posix.Terminal (queryTerminal)
+import System.Posix.IO (stdOutput)
+#endif
 
 type Transform = Pandoc -> Pandoc
 
@@ -1357,7 +1361,12 @@ convertWithOpts opts args = do
                       , readerFileScope   = fileScope
                       }
 
-  when (not (isTextFormat format) && outputFile == "-") $
+#ifdef _WINDOWS
+  let istty = True
+#else
+  istty <- queryTerminal stdOutput
+#endif
+  when (istty && not (isTextFormat format) && outputFile == "-") $
     err 5 $ "Cannot write " ++ format ++ " output to stdout.\n" ++
             "Specify an output file using the -o option."
 
@@ -1457,8 +1466,9 @@ convertWithOpts opts args = do
            applyTransforms transforms >=>
            applyFilters datadir filters' [format]) doc
 
-  let writeBinary :: B.ByteString -> IO ()
-      writeBinary = B.writeFile (UTF8.encodePath outputFile)
+  let writeFnBinary :: FilePath -> B.ByteString -> IO ()
+      writeFnBinary "-" = B.putStr
+      writeFnBinary f   = B.writeFile (UTF8.encodePath f)
 
   let writerFn :: FilePath -> String -> IO ()
       writerFn "-" = UTF8.putStr
@@ -1466,7 +1476,7 @@ convertWithOpts opts args = do
 
   case writer of
     IOStringWriter f -> f writerOptions doc' >>= writerFn outputFile
-    IOByteStringWriter f -> f writerOptions doc' >>= writeBinary
+    IOByteStringWriter f -> f writerOptions doc' >>= writeFnBinary outputFile
     PureStringWriter f
       | pdfOutput -> do
               -- make sure writer is latex or beamer or context or html5
@@ -1486,7 +1496,7 @@ convertWithOpts opts args = do
 
               res <- makePDF pdfprog f writerOptions doc'
               case res of
-                   Right pdf -> writeBinary pdf
+                   Right pdf -> writeFnBinary outputFile pdf
                    Left err' -> do
                      B.hPutStr stderr err'
                      B.hPut stderr $ B.pack [10]