added the an offsets option for IndexOptions without it users can not use the postings highlighter see issue 891

Author: dev-teresab

Diff for: src/Nest/Enums/IndexOptions.cs

@@ -12,6 +12,8 @@ public enum IndexOptions
 		[EnumMember(Value = "freqs")]
 		Freqs,
 		[EnumMember(Value = "positions")]
-		Positions
+		Positions,
+        [EnumMember(Value = "offsets")]
+        Offsets,
 	}
 }