Convert decoder to shared npm library

[cavis]

There's a fair amount of overlap between this repo's decoder and the analytics-ingest-lambda inputs. They need to take in variable kinesis-y inputs, and turn that into "log-lines/records". Though with CloudFront inputs, the format may vary a bit, and needs to be configurable. But this feels ripe for conversion to a shared library, as we start thinking about breaking apart the analytics-ingest-lambda.

Support inputs:

• Unencoded kinesis records (try JSON)
• CloudWatch log subscription gzip+base64 lines (try JSON)
• CloudFront realtime logs (configurable)
• CloudFront standard logs (delivered through ??? kinesis ???)