Bug Fixing - IsSelected is not implemented in ResultItemViewModel

If this property is not implemented, the Theme of selected search result
won't take effect.

Wox/ViewModel/ResultItemViewModel.cs

@@ -14,6 +14,7 @@ namespace Wox.ViewModel
         #region Private Fields
 
         private Result _result;
+        private bool _isSelected;
 
         #endregion
 
@@ -77,6 +78,16 @@ namespace Wox.ViewModel
             }
         }
 
+        public bool IsSelected
+        {
+            get { return _isSelected; }
+            set
+            {
+                _isSelected = value;
+                OnPropertyChanged("IsSelected");
+            }
+        }
+
         public RelayCommand OpenResultCommand
         {
             get;