Render specific status codes (issue: #519)

[xt-riot]

In response to #519

Added the optional argument renderOnlyStatusCode. This argument needs the renderStatusCodes to be set. validate.js takes the value of the argument and creates an array(by either splitting the value, which handles multiple comma-separated status codes, or just using the single code supplied)

Usage

node ./autocannon.js -c 1 -d 2 --renderStatusCodes "http://localhost:3000/test3"

node ./autocannon.js -c 1 -d 2 --renderOnlyStatusCode=404 --renderStatusCodes "http://localhost:3000/test3"

node ./autocannon.js -c 1 -d 2 --renderOnlyStatusCode=200,302 --renderStatusCodes "http://localhost:3000/test3"