Add src-filename and dest-filename template variables

Closes jgm#3431.

MANUAL.txt

@@ -1072,6 +1072,12 @@ Some variables are set automatically by pandoc.  These vary somewhat
 depending on the output format, but include metadata fields as well
 as the following:
 
+`src-filename`, `dest-filename`
+:   source and destination filenames as given on the command line.
+    `source-filename` can also be a list if multiple input files are given,
+    or `(stdin)` if the input comes through a pipe.  Similarly, `dest-filename`
+    can be `(stdout)` if the output goes to the terminal.
+
 `title`, `author`, `date`
 :   allow identification of basic aspects of the document.  Included
     in PDF metadata through LaTeX and ConTeXt.  These can be set

src/Text/Pandoc/App.hs

@@ -229,8 +229,18 @@ convertWithOpts opts = do
   let withList _ [] vars = return vars
       withList f (x:xs) vars = f x vars >>= withList f xs
 
+  let srcFilenames = if null $ optInputFiles opts
+                     then ["(stdin)"] else optInputFiles opts
+  let destFilename = if optOutputFile opts == "-"
+                     then "(stdout)"  else optOutputFile opts
+
   variables <- return (optVariables opts)
       >>=
+      (\vars -> return $ ("dest-filename", destFilename) : vars)
+      >>=
+      withList (addStringAsVariable "src-filename")
+               (srcFilenames)
+      >>=
       withList (addContentsAsVariable "include-before")
                (optIncludeBeforeBody opts)
       >>=