Add stop-on-rate-limit flag

[ethanannane]

Not polished at all yet, would like to know what I can do to improve it.
Added a check in the ProcessWord function that looks similar to this:

if statusCode == http.StatusTooManyRequests && d.globalopts.StopOnRateLimit { fmt.Println("Rate limit reached, exiting...") os.Exit(0)	}