Add escape hatch for Series merging

This is an universal solution for plugins to override how series are merged.
The reason to override is so we can set the same provider id on multiple items without merging them, while using another id for merging them. Having an (optional) provider id not tied to any online database allows plugins to use their own rules for merging series.

MediaBrowser.Controller/Entities/TV/Series.cs

@@ -184,6 +184,11 @@ namespace MediaBrowser.Controller.Entities.TV
                 list.Insert(0, key);
             }
 
+            if (this.TryGetProviderId(MetadataProvider.Custom, out key))
+            {
+                list.Insert(0, key);
+            }
+
             return list;
         }
 

MediaBrowser.Model/Entities/MetadataProvider.cs

@@ -7,6 +7,12 @@ namespace MediaBrowser.Model.Entities
     /// </summary>
     public enum MetadataProvider
     {
+        /// <summary>
+        /// This metadata provider is for users and/or plugins to override the
+        /// default merging behaviour.
+        /// </summary>
+        Custom = 0,
+
         /// <summary>
         /// The imdb.
         /// </summary>