Read from STDIN if data is being piped in

```javascript
if (process.stdin.isTTY) {
  cli.addArgument(['file'], {
    help:   'File to read, utf-8 encoded without BOM'
  });
}
```

...was a cheeky way of making `file` optional (I couldn't figure out how to make positional arguments optional with `argparse`).  However, a side-effect of this is that `-` isn't a valid way to instruct `js-yaml` to read from `STDIN`.  I'm happy to amend the patch if you have suggestions on how to achieve this.