fromString option, use index from argument array for filename instead of "?"

The index allows the caller to map things like parse errors back to the
code chunk where they appeared.

tools/node.js

@@ -73,13 +73,13 @@ exports.minify = function(files, options) {
     } else {
         if (typeof files == "string")
             files = [ files ];
-        files.forEach(function(file){
+        files.forEach(function(file, i){
             var code = options.fromString
                 ? file
                 : fs.readFileSync(file, "utf8");
             sourcesContent[file] = code;
             toplevel = UglifyJS.parse(code, {
-                filename: options.fromString ? "?" : file,
+                filename: options.fromString ? i : file,
                 toplevel: toplevel
             });
         });