Feature request: Custom formatter for sdjournal reader

[Richterrettich]

At the moment, JournalReader has a fixed message template for each entry. It would be really helpful if I would be able to alter the format.

I have thought of something like this:

config := sdjournal.JournalReaderConfig {
		Formatter: func(entry *JournalEntry) string {
                     return fmt.Sprintf("%s:%s",entry.Fields["MY_CUSTOM_FIELD"])
                },
	}

I can send you a patch if you are okay with this kind of feature.