Added exit func in echo.cpp

@@ -5,10 +5,14 @@ using namespace std;
 int main(){
     string str;
     while(1){
-        cout << "Implements echo program\nInput >> ";
+        cout << "Implements echo program\n";
+        cout << "If you want exit, Input \"exit\" : ";
         cin >> str;
-        cout << "Output >> " << str << endl;
+        if(str == "exit"){
+            exit(100);
+        }
+        else cout << "Output >> " << str << endl;
     }
-
     return 0;
 }
+