The date format is now selectable.

Added the form controls and AppSettings keys to allow the user to select
the format of the date appended to the output file.
Further update to issue #1.

MergePDFs.cs

@@ -81,7 +81,8 @@
         public static string GetDate()
         {
             DateTime date = DateTime.Now;
-            return date.ToString("yyyy-MM-dd");
+            string dateFormat = ConfigurationManager.AppSettings["dateFormat"];
+            return date.ToString(dateFormat);
         }
     }