Check that messages older than two weeks are not passed to bulk delete

This resolves #477

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -181,5 +181,13 @@ namespace Discord
             if (msg == null) return new ArgumentException($"Value must be less than {value}", name);
             else return new ArgumentException(msg, name);
         }
+
+        // Bulk Delete
+        public static void NoMessageOlderThanTwoWeeks(ulong[] collection, string name)
+        {
+            var minimum = DateTimeUtils.ToSnowflake(DateTimeOffset.Now.Subtract(TimeSpan.FromDays(14)));
+            for (var i = 0; i < collection.Length; i++)
+                if (collection[i] <= minimum) throw new ArgumentOutOfRangeException(name, "Messages must be younger than two weeks to delete.");
+        }
     }
 }