Add support for priority

Set default title to topic unique, as displayed in subscriptions list, if not provided
Increase toast timeout to 5 seconds

ntfysh_client/Notifications/NotificationPriority.cs

@@ -0,0 +1,11 @@
+namespace ntfysh_client.Notifications
+{
+    public enum NotificationPriority
+    {
+        Min = 1,
+        Low = 2,
+        Default = 3,
+        High = 4,
+        Max = 5
+    }
+}