Allow node.js to exit naturally

Calling `process.exit()` can cause process to abort before the entire text
is written to the stdout.

When I tested this, out of 1.8Mb binary only 65536 bytes had been written.

bin/markdown-it.js

@@ -100,6 +100,4 @@ readFile(options.file, 'utf8', function (err, input) {
   }
 
   process.stdout.write(output);
-
-  process.exit(0);
 });