Update formatting

[stephen-soltesz]

This change updates the formatting for readability. This change also adds a third average rate for the total average time period. For multiple streams, three averages are reported now. This addition is for visibility and experimentation.

For example:

$ minimal-download -duration 1s -streams 4
Connecting: wss://msak-mlab2-lga04.mlab-oti.measurement-lab.org/throughput/v1/download?...
Download stream #1 rate: 404.37 Mbps, MinRTT  5.45ms, elapsed 0.2801s, application r/w: 0/14158080
Download stream #1 rate: 458.88 Mbps, MinRTT  4.46ms, elapsed 0.6034s, application r/w: 0/34611752
Download stream #1 rate: 475.83 Mbps, MinRTT  4.46ms, elapsed 0.7935s, application r/w: 0/47196737
Download stream #1 rate: 483.44 Mbps, MinRTT  4.46ms, elapsed 1.2671s, application r/w: 0/76571443
Download stream #1 rate: 483.42 Mbps, MinRTT  4.46ms, elapsed 1.2672s, application r/w: 0/76572491
------
Download total average:  483.41 Mbps, MinRTT  4.46ms, elapsed 1.2672s, application r/w: 0/76572491
Download first average:  482.17 Mbps, MinRTT  4.46ms, elapsed 1.1486s, application r/w: 0/69225165
Download center average: 486.65 Mbps, MinRTT  4.46ms, elapsed 1.1376s, application r/w: 0/69199565

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7497566946

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 70.216%

---

Totals
Change from base Build 7496181239:	0.0%
Covered Lines:	877
Relevant Lines:	1249

---

💛 - Coveralls

[robertodauria]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained