feature: 95th percentile in latency statistics

[a053a]

Using the 95th percentile is common among performance measurements. This feature will allow bombardier to output the 95th percentile within the latency statistics and json output.

Example of latency statistics:

# ./bombardier -n 100 -c 1 http://www.google.ca -l
Statistics        Avg      Stdev        Max
  Reqs/sec        12.58      67.10    1470.52
  Latency      100.21ms   130.73ms      1.21s
  Latency Distribution
     50%    75.40ms
     75%    79.78ms
     90%    96.40ms
     95%   148.84ms
     99%   577.28ms
  HTTP codes:
    1xx - 0, 2xx - 100, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:   122.63KB/s

Example of JSON output:

# ./bombardier -n 100 -c 1 http://www.google.ca -o j
{
  "spec": {
    "numberOfConnections": 1,
    "testType": "number-of-requests",
    "numberOfRequests": 100,
    "method": "GET",
    "url": "http://www.google.ca",
    "body": "",
    "stream": false,
    "timeoutSeconds": 2,
    "client": "fasthttp"
  },
  "result": {
    "bytesRead": 1251011,
    "bytesWritten": 6222,
    "timeTakenSeconds": 21.252998768,
    "req1xx": 0,
    "req2xx": 100,
    "req3xx": 0,
    "req4xx": 0,
    "req5xx": 0,
    "others": 0,
    "latency": {
      "mean": 212518.7,
      "stddev": 1167030.4331027067,
      "max": 11809446
    },
    "rps": {
      "mean": 4.829551616738988,
      "stddev": 14.50170391245913,
      "max": 116.41387974748899,
      "percentiles": {
        "50": 0,
        "75": 0,
        "90": 0,
        "95": 49.924945,
        "99": 58.68506
      }
    }
  }
}

[codesenberg]

Yes, sure. Let's add it. 👍