Implement some log format functions in this package?

[fredrikekre]

LoggingExtras has two loggers that accept a function (io, args) -> message for formatting (FormatLogger and DatetimeRotatingFileLogger). In LokiLogger I used the same interface. In that package I also provide two pre-defined format functions json for formatting the message as JSON, and logfmt for formatting as logfmt.

I feel like these type of pre-defined formatting function can be useful to others. I thought about either creating a separate LoggingFormats package, or to include it in this package. In addition to the ones I mentioned it could make sense to also (re)define the formatting from ConsoleLogger and SimpleLogger perhaps. Thoughts?

[racinmat]

I really like this idea, I have truncating logger in #56, which I think would be useful too.

[fredrikekre]

https://github.com/JuliaLogging/LoggingFormats.jl