Custom EOL option for FileTransport

I've some users on Windows, and when I ask them to open the logs, they usually double click (launching notepad.exe) which doesn't handle very well the `\n` thing.

I am planning to use with this:

```
winston.add(winston.transports.File, {
  //...options
  eol: require('os').EOL
});
```

Thanks
We could tack this for `1.0.0` then to be safe. Ill open an issue.