Request cancellations support added to Middlewares.

Some core functions refactored as middlewares.
Fixed race condition in exporting system. Now, only one goroutine will be responsible for exporting. This fixes concurrency issues on writing.

request.go

@@ -7,8 +7,26 @@ import (
 // Request is a small wrapper around *http.Request that contains Metadata and Rendering option
 type Request struct {
 	*http.Request
-	Meta     map[string]interface{}
-	Rendered bool
+	Meta      map[string]interface{}
+	Rendered  bool
+	Cancelled bool
+}
+
+func allowedDomainsMiddleware(g *Geziyor, r *Request) {
+	if len(g.Opt.AllowedDomains) != 0 && !contains(g.Opt.AllowedDomains, r.Host) {
+		//log.Printf("Domain not allowed: %s\n", req.Host)
+		r.Cancelled = true
+		return
+	}
+}
+
+func duplicateRequestsMiddleware(g *Geziyor, r *Request) {
+	if !g.Opt.URLRevisitEnabled {
+		if _, visited := g.visitedURLs.LoadOrStore(r.Request.URL.String(), struct{}{}); visited {
+			//log.Printf("URL already visited %s\n", rawURL)
+			r.Cancelled = true
+		}
+	}
 }
 
 func defaultHeadersMiddleware(g *Geziyor, r *Request) {
@@ -24,3 +42,13 @@ func headerSetDefault(header http.Header, key string, value string) http.Header
 	}
 	return header
 }
+
+// contains checks whether []string contains string
+func contains(s []string, e string) bool {
+	for _, a := range s {
+		if a == e {
+			return true
+		}
+	}
+	return false
+}