Allow using stdin

Your test `file_exists('php://stdin')` returns `FALSE`, and `file_get_contents` doesn’t work with `/dev/stdin` or `-`. I suggest that you

1. get rid of the check (`file_get_contents` will complain with a better message anyway if it can’t read the file), and
2. do `if ($options['input'] === '-') $options['input'] = 'php://stdin';` for a more natural command line interface