Add stringer support for sub-logger

[ccamel]

Same than #185 but for zerolog.Context.

Enables the use of stringerfor sub-loggers:

	sublogger := log.With().
		Str("component", "foo").
		Stringer("ip", net.IP{127,0,0,1}).
		Logger()
	
	sublogger.Info().Msg("hello world")