Add option to output to stdout or stderr in AppenderConsole

[gadenbuie]

The PR is motivated by trying to use lgr inside R Markdown documents, where it's generally better to send logging messages to stderr so they aren't captured in the document output.

This PR adds an output parameter to AppenderConsole$new() that is NULL by default but takes either "stdout" or "stderr". In the default case, the preference is for "stdout" unless running in a knitr rendering process (signaled by the knitr.in.progress option being TRUE). If a user really wants to output to stdout inside knitr, they can use AppenderConsole$new(output = "stdout").

[s-fleck]

Thanks! I'll try to review it asap, looks good!

[s-fleck]

Hey sorry it took me a while. I reworked the feature to make it more flexible by allowing arbitrary connections as output for AppenderConsole, and some other adaptions to make it fit a bit better into the package.