When enabled, lines like "# interval-start 1234567890123456" and "#
interval-end 1234567890123456" will be added to the data output. Note
that there may be an "interval-end" timestamp followed by an identical
"interval-start" timestamp, if the response at the nilmdb level was
split up into multiple chunks.
In general, assume contiguous data if previous_interval_end ==
new_interval_start.
