feature: custom log targets (fluentd)

[jasonswearingen]

allow logger to direct output to https://github.com/fluent/fluent-logger-node

allow any fluentd target as a recipient.

this will allow projects to customize log recipients

Also as a bonus, projects running on google-cloud will have automatic log export to stackdriver logging. see: https://cloud.google.com/error-reporting/docs/setup/compute-engine#using_logging