Custom Import List improvements

Fixed: Add placeholder title for Custom Import List title
New: Support 'title' property for Custom Import List

src/NzbDrone.Core/ImportLists/Custom/CustomAPIResource.cs

@@ -2,6 +2,7 @@ namespace NzbDrone.Core.ImportLists.Custom
 {
     public class CustomSeries
     {
+        public string Title { get; set; }
         public int TvdbId { get; set; }
     }
 }

src/NzbDrone.Core/ImportLists/Custom/CustomImport.cs

@@ -43,6 +43,7 @@ namespace NzbDrone.Core.ImportLists.Custom
                 {
                     series.Add(new ImportListItemInfo
                     {
+                        Title = item.Title.IsNullOrWhiteSpace() ? $"TvdbId: {item.TvdbId}" : item.Title,
                         TvdbId = item.TvdbId
                     });
                 }