Fluent config should not be in source code

[nna774]

https://github.com/drobune/goluent/blob/8a1750deea3eaf3b8aaf71ce04d5c82691c5b4d8/goluent.go#L32-L34

get from rom config file or environment is more good.

[ychsn]

👍 thx!