Add SetRecordKeyNames to Logger interface

[neilisaac]

Allow applications to override the common key names (msg/lvl/t).

This is useful for some log aggregation services which only parse specific field names. LogDNA for example only parses "level" and "message" keys.

Usage example:

logger := log15.New()
logger.SetRecordKeyNames(log15.RecordKeyNames{
    Time: "timestamp", Msg: "message", Lvl: "level"})

Fixes https://github.com/inconshreveable/log15/issues/147