Default to $stdout with optional override

I think the original `$stdout` was definitely a better default, that way people can continue capturing the output using well know ruby idioms.

Most examples of capturing `stdout` essentially look like this.
```
 def capture_stdout
    out = StringIO.new
    $stdout = out
    yield
    return out
  ensure
    $stdout = STDOUT
  end
```

The recent change to use a non-overridable `STDOUT` would force people to redefine the constant, or monkey patch the `LogstashLogger::Device::Stdout` class.

I think this change is the best of both worlds. It has a good default, but remains flexible for people like myself who needs to deal with the fact that somebody has overridden `$stdout`.