Added support to open file when double clicked in explorer.

ucalc/MainWindow.xaml.cs

@@ -24,7 +24,12 @@ namespace UCalc
         {
             base.OnContentRendered(e);
 
-            if (_showRecover)
+            var args = Environment.GetCommandLineArgs();
+            if (args.Length > 1)
+            {
+                OpenBilling(args[1]);
+            }
+            else if (_showRecover)
             {
                 _showRecover = false;