[enh] add Abort function to Request

colly.go

@@ -353,6 +353,10 @@ func (c *Collector) scrape(u, method string, depth int, requestData io.Reader, c
 
 	c.handleOnRequest(request)
 
+	if request.abort {
+		return nil
+	}
+
 	if method == "POST" && req.Header.Get("Content-Type") == "" {
 		req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
 	}

request.go

@@ -25,6 +25,12 @@ type Request struct {
 	// ID is the Unique identifier of the request
 	ID        uint32
 	collector *Collector
+	abort     bool
+}
+
+// Abort cancels the HTTP request when called in an OnRequest callback
+func (r *Request) Abort() {
+	r.abort = true
 }
 
 // AbsoluteURL returns with the resolved absolute URL of an URL chunk.