Add Random Delay for Limit Rule

Description:

    Added random delay config to limitRule that allow user to specify additional parameter to
        randomized the delay on each request.

    Added an example for random delay

Testing Methodogy:

    Used the rate_limit app in the _example dir and verify that adding RandomDelay will cause
        each request to have different delay time.

_examples/random_delay/random_delay.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/gocolly/colly"
+	"github.com/gocolly/colly/debug"
+)
+
+func main() {
+	url := "https://httpbin.org/delay/2"
+
+	// Instantiate default collector
+	c := colly.NewCollector()
+
+	// Attach a debugger to the collector
+	c.SetDebugger(&debug.LogDebugger{})
+
+	// Limit the number of threads started by colly to two
+	// when visiting links which domains' matches "*httpbin.*" glob
+	c.Limit(&colly.LimitRule{
+		DomainGlob:  "*httpbin.*",
+		Parallelism: 2,
+		RandomDelay: 5 * time.Second,
+	})
+
+	// Start scraping in four threads on https://httpbin.org/delay/2
+	for i := 0; i < 4; i++ {
+		go c.Visit(fmt.Sprintf("%s?n=%d", url, i))
+	}
+	// Start scraping on https://httpbin.org/delay/2
+	c.Visit(url)
+	// Wait until threads are finished
+	c.Wait()
+}

http_backend.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"net/http"
 	"net/http/cookiejar"
 	"os"
@@ -37,6 +38,8 @@ type LimitRule struct {
 	DomainGlob string
 	// Delay is the duration to wait before creating a new request to the matching domains
 	Delay time.Duration
+	// RandomDelay is the extra randomized duration to wait added to Delay before creating a new request
+	RandomDelay time.Duration
 	// Parallelism is the number of the maximum allowed concurrent requests of the matching domains
 	Parallelism    int
 	waitChan       chan bool
@@ -148,7 +151,12 @@ func (h *httpBackend) Do(request *http.Request, bodySize int) (*Response, error)
 	if r != nil {
 		r.waitChan <- true
 		defer func(r *LimitRule) {
-			time.Sleep(r.Delay)
+			randomDelay := time.Duration(0)
+			rand.Seed(time.Now().UnixNano())
+			if r.RandomDelay != 0 {
+				randomDelay = time.Duration(rand.Intn(int(r.RandomDelay)))
+			}
+			time.Sleep(r.Delay + randomDelay)
 			<-r.waitChan
 		}(r)
 	}