Allow for additional fields to be set in the config file, (as per gelf4net)

[AGBrown]

gelf4net (the log4net graylog appender) allows for additional fields to be set in the config file. It would be very useful if this dll did that too.

I will provide a PR as soon as I can.

[AGBrown]

PR submitted, and it works fine. Note I wasn't able to work out how to get layout renderers to work with the config properties, e.g.

AdditionalFields="app:RandomSentence,version:1.0,Level:${level}"

This is mainly as I have no idea how those layout renderers work.

The ShouldSendLongUdpMessage test fails, but I don't think that is a result of what I've done (it was failing on 5e2f37e1 on my machine anyway)

[tobiaszuercher]

in my eyes a better approach: https://github.com/2020Legal/NLog.Targets.Gelf