[cli] Don't log to STDOUT if debug flags are set

Created by: devplayer0

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

It makes sense that error messages should be written to STDERR and all others should be written to STDOUT (as shown in #207). However, it would be convenient to parse the debugging output when the relevant flags are set.

This change will disable logging to STDOUT and redirect all log messages to STDERR when any of the debug flags are set. (Resolves #473 (closed))