Write out a string with 'null' for empty value list entries
Update fixme with gh issue id. Remove no longer relevant fixme
Support multiple data sources. Emit real numerical values.

 - Iterate through all the data sources, and build up a time series to
   send to InfluxDB. This allows plugins like load + interface to
   properly emit muliple values (e.g. load/load/shortterm,middterm,longterm)
 - Add function to emit a single value as JSON.
 - Extend format_json_name to inject the data source name if != "value"