Add flag to allow choosing between stdout, stderr for log output

[atc0005]

Currently the app is hard-coded to write to stdout. I suspect I did this to simplify things, but it shouldn't be hard to do a light refactor to add a flag with a default value of either stdout or stderr (which I think is the original default). The atc0005/bounce project is already doing this, albeit a little less cleaner than I'd like.