Add KAFKA_CONFIGS variable to do additional configuration

[raarts]

I needed more specific per-topic settings.
This PR allows using the kafka-configs.sh script during creation time.

[sscaling]

This is already supported. Please see example in https://github.com/wurstmeister/kafka-docker/blob/master/test/0.10/test.create-topics.kafka.sh#L16

[raarts]

I see the test, but I can't explain how it works, where is the extra --config parameter parsed in create-config.sh (looking at https://github.com/wurstmeister/kafka-docker/blob/master/create-topics.sh)

Does kafka-topics.sh accept multiple --config arguments? I remember trying and it did not work.
Also, this isn't documented in the readme.

EDIT: ok, I think I understand now. Should be documented better.

[sscaling]

Yes - admittedly it's not obvious, but implemented to support backwards compatibility.

test:2:1:compact --config=<x> --config=<y> --config=<z>

Where compact is topic compaction policy. Documentation of this is an existing issue: #311

[sscaling]

ok, I think I understand now. Should be documented better.

happy to close this PR?

[sscaling]

Thanks!