Add dynamic HEC metadata support based on record Introspection

[dtregonning]

Customers are looking to add custom source, sourcetype, host to HEC payloads based of a field or value stored within a Kafka Message.

We should add-support for dynamic HEC meta-data.

[dtregonning]

We should look at leveraging Kafka Headers to achieve this:

Will be fully available and released in Kafka 1.1

[hire-vladimir]

would be neat to also specify special semi-static fields, ie splunk_kafka_processor_server that would default to the node name that processed the read event from the bus. This is very handy when it comes to troubleshooting/understanding load balancing/work distribution & issue tracking.

[dtregonning]

fixed in #168