only append extra_args if it's defined

Otherwise Emscripten's JS chokes; it assumes that every element of the
argv array is a string, and converts them into an array of bytes for
the running program to use. Fixes #28.

loader.js

@@ -682,7 +682,9 @@ var Module = null;
        }
      }
 
-     args = args.concat(extra_args);
+     if (extra_args) {
+       args = args.concat(extra_args);
+     }
 
      var path = emulator_start.split(/\\|\//); // I have LTS already
      args.push('-c', /^[a-zA-Z]:$/.test(path[0]) ? path.shift() : 'c:');