Add support for P90 in Metrics

[fxkr]

Background

Closes #28

To align with other systems like Prometheus which provides only P90, and got no P95.

Example output:

% go run github.com/tsenart/vegeta report results.bin
Requests      [total, rate, throughput]    39709, 7940.90, 302.50
Duration      [total, attack, wait]        30.162362361s, 5.000565258s, 25.161797103s
Latencies     [mean, 50, 90, 95, 99, max]  625.208116ms, 60.995µs, 3.991791ms, 9.797688ms, 30.00028521s, 30.026509326s
Bytes In      [total, mean]                2709828, 68.24
Bytes Out     [total, mean]                0, 0.00
Success       [ratio]                      22.98%
Status Codes  [code:count]                 0:30585  200:9124  
Error Set:
.....

# go run github.com/tsenart/vegeta report -type=json -buckets '[0,5ms]' results.bin|jq
{
  "latencies": {
    "total": 24826389112876,
    "mean": 625208116,
    "50th": 60995,
    "90th": 3991791,
    "95th": 9797688,
    "99th": 30000285210,
    "max": 30026509326
  },
 ...
}

Checklist

• Git commit messages conform to community standards.
• Each Git commit represents meaningful milestones or atomic units of work.
• Changed or added code is covered by appropriate tests.

[tsenart]

Looks good to me, but we need to update a few other places too. Can you please do that?

• https://sourcegraph.com/github.com/tsenart/vegeta@6159efd476eb774021d9d1f42e996b20b0d222d1/-/blob/lib/metrics_test.go#L115
• https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/tsenart/vegeta%24+99th&patternType=literal

[fxkr]

Done.

Also fixed typo "an" -> "and" in the same line in README.

The 90th in README is made up as I don't have the original results.bin.

I don't really see why the benchmark needs the P90, but added as requested.

Also rebased to master.