add args3

@@ -670,6 +670,15 @@ string SrsConfDirective::arg2()
     return "";
 }
 
+string SrsConfDirective::arg3()
+{
+    if (args.size() > 3) {
+        return args.at(3);
+    }
+    
+    return "";
+}
+
 SrsConfDirective* SrsConfDirective::at(int index)
 {
     srs_assert(index < (int)directives.size());

@@ -201,6 +201,7 @@ public:
     virtual std::string arg0();
     virtual std::string arg1();
     virtual std::string arg2();
+    virtual std::string arg3();
 // directives
 public:
     /**