Add download stats and estimate to fetch-data (#149)

[willkg]

This adds a --stats option to fetch-data that will show download stats and an estimate instead of the verbose log of what it downloaded.

With --no-stats (the default):

$ supersearch --num=105 | fetch-data --processed --no-raw --no-dumps --overwrite --workers=10 --no-stats crashdata
Using API token: xxx
Using 10 workers.
e96fe956-3c2f-4d58-b9b9-30d200240918: fetching processed crash
... CLIP ...
191c49a6-22a0-4d1f-a606-2c8f00240918: fetching processed crash
Completed in 0:00:05.

With `--stats``:

$ supersearch --num=105 | fetch-data --processed --no-raw --no-dumps --overwrite --workers=10 --stats crashdata
Using API token: xxx
Using 10 workers.
Downloaded (0/105) 0:00:52.308396
Downloaded (100/105) 0:00:00.514721
Completed in 0:00:08.