Add option to allow logging to standard error

[Georift]

Sometimes you'd like to be able to influence the log lines and send some logs to standard error. This option add the property LOG_TO_STANDARD_ERROR which will send any failing status codes to standard error.

To confirm it's working:

docker run --rm -e LOG_WITHOUT_NEWLINE=true -e LOG_TO_STANDARD_ERROR=true -p 8080:8080 mendhak/http-https-echo:latest 2> errors

Will output only standard error to the errors file.

Test it logs to standard error:

curl -v -H "x-set-response-status-code: 401" http://localhost:8080/

Expect to see a log line appear in errors but not on the docker run output

Test it logs to standard out:

curl -v -H "x-set-response-status-code: 350" http://localhost:8080/

Expect to see a log line appear in the docker run output, but not in errors

[mendhak]

Sorry for late reply, are you able to add a test here: https://github.com/mendhak/docker-http-https-echo/blob/master/tests.sh

for what you've added.