Major changes:
* Remove intermediate batching of kinesis records
* Call the callback func with each record
* Use functional options for config
https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
Minor changes:
* update README messaging about Kinesis -> Firehose functionality
* remove unused buffer and emitter code
To allow other checkpoint backends we extract a checkpoint interface
which future checkpoints can implement.
* Add checkpoint interface for custom checkpoints
* Create RedisCheckpoint to implement checkpoint interface
* Swap out hosie redis library for go-redis
Minor changes
* Allow configuration of Redis endpoint with env var `REDIS_URL`
* Replace gvt with govendor
