Allow whitelisting Kafka topics

[NeQuissimus]

I am running a snapshot Docker image of this right now, everything seems to work fine using the following application.conf

kafka-lag-exporter {
  clusters = [
    {
      name = "dev"
      bootstrap-brokers = "broker1:9092,broker2:9092,broker3:9092"
      consumer-properties = {
        client.id = "kafka-lag"
      }
      admin-client-properties = {
        client.id = "kafka-lag-admin"
      }
    }
  ]
  lookup-table-size = 200
  client-group-id = "test-kafkalagexporter"
  metric-whitelist = ["kafka_consumergroup_group_lag_seconds"]
  topic-whitelist = ["prefix1-.*", "prefix2-.*"]
}

[seglo]

Hi Tim. Thanks for the PR. This along with #4 and #43 will provide a nice set of options for users to limit the number of metrics reported.

I have a few requests:

1. Please add a test to KafkaClientSpec.
2. I assume you probably don't use Helm, but would you mind adding the configuration to the Helm chart too in ./charts/kafka-lag-exporter/. It should be easy to follow the same convention as metricWhitelist in the values.yaml and templates/030-ConfigMap.yaml. Don't worry about testing it, I can do that.
3. Add the configuration to the Helm configuration section in the README.md.

[NeQuissimus]

Please check the changes to the Helm chart.
I added a few tests for different edge cases.

[seglo]

LGTM. Thanks a lot for the extra test coverage!