Add new priority level to ResolverPriority for plugins

MediaBrowser.Controller/Resolvers/ResolverPriority.cs

@@ -5,6 +5,11 @@ namespace MediaBrowser.Controller.Resolvers
     /// </summary>
     public enum ResolverPriority
     {
+        /// <summary>
+        /// The highest priority. Used by plugins to bypass the default server resolvers.
+        /// </summary>
+        Plugin = 0,
+
         /// <summary>
         /// The first.
         /// </summary>