Add `json_fields` parameter to set_up_logging
Add `json_fields` parameter to set_up_logging

This will allow the main function to add extra fields to JSON log
messages, for example to pass through command-line arguments.

See https://www.pivotaltracker.com/story/show/70748012

alphagov/performanceplatform-collector,alphagov/performanceplatform-collector,alphagov/performanceplatform-collector