Set flags for individual logger

[pilagod]

Set flags now only allow for default logger, it would be nice to set flags for individual logger.

[adjackura]

I'm a little confused by you comment, what do you mean 'only allow for default logger'? Do you mean a single Logger as the default logger? I'm confused as the package also has the notion of a default logger and you can't set those flags.
You PR allows you to set flags for all the sub loggers, do you mean a SetFlags for each of those as well (like SetFlagsWarning and SetFlagsInfo?)

[pilagod]

Hello @adjackura, sorry for the confusion.

What I want is to be able to customize the format for the messages (inspired by #18). According to second usage example in README, we can setup multiple logger instances through Init. #18 enable us to customize format of messages for the first created logger instance (a.k.a. defaultLogger), and I hope we can customize messages for other logger instances created after the first one.

The logger I mention in previous comment refers to logger instance, which returned from Init, not sub loggers in one logger instance.

Hope this make it clearer, thanks !