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)

		}