Allow for setting user fields to enrich the JSON documents

[rtoma]

Imagine adding config like:

redislog.userFields = "field1:value1[,field2:value2]"

Which will result in these fields being added to each JSON document, like:

{
  "@timestamp": "2015-10-20T15:13:58.042+0200",
  "process_id": 7240,
  "session_id": "56263e15.1c48",
  "session_line_num": 1,
  "session_start_time": "2015-10-20T15:13:57+0200",
  "error_severity": "LOG",
  "hint": "Future log output will appear in directory \"pg_log\".",
  "message": "redirecting log output to logging collector process"
  "field1": "value1", <---
  "field2": "value2", <---
}

This enhancement makes it possible to enrich the JSON docs with properties like: environment, database name, instance name et cetera...

Have fun in Vienna next week!