GELF support

[mikaelstaldal]

GELF is a kind-of standardized JSON based log event format, it would be useful if Penna could generate it.

It is not only used by Graylog, but can also be ingested by by e.g. Logstash.

Other logging frameworks can produce it, e.g. Log4j 2 and Rainbow Gum.

[hkupty]

I'll look into it. Seems good to root Penna to some standard and this might be a good choice. I'd expect this to land in next "major" bump, either 0.10 or 1.0.

[hkupty]

Also, might supersede #43

[mikaelstaldal]

I am not sure if it's good to have GELF as the only format though, since it is arguably not optimal to convey the information from a SLF4J log event.

[hkupty]

Then I think I'll need different backends for different formats, which might cause some minor internal rearranging, but it's definitely doable

[mikaelstaldal]

Yes, it's probably wise to support different output formats, and perhaps user defined formats as well for those who use programmatic configuration.