Add sourcefile and outputfile template variables (#3439)

Closes #3431.

MANUAL.txt

@@ -1074,6 +1074,22 @@ Some variables are set automatically by pandoc.  These vary somewhat
 depending on the output format, but include metadata fields as well
 as the following:
 
+`sourcefile`, `outputfile`
+:   source and destination filenames, as given on the command line.
+    `sourcefile` can also be a list if input comes from multiple files, or empty
+    if input is from stdin. You can use the following snippet in your template
+    to distinguish them:
+
+        $if(sourcefile)$
+        $for(sourcefile)$
+        $sourcefile$
+        $endfor$
+        $else$
+        (stdin)
+        $endif$
+
+    Similarly, `outputfile` can be `-` if 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

@@ -230,6 +230,11 @@ convertWithOpts opts = do
 
   variables <- return (optVariables opts)
       >>=
+      (\vars -> return $ ("outputfile", optOutputFile opts) : vars)
+      >>=
+      withList (addStringAsVariable "sourcefile")
+               (optInputFiles opts)
+      >>=
       withList (addContentsAsVariable "include-before")
                (optIncludeBeforeBody opts)
       >>=