Add Async version of insert multiple rows

[NoamWe]

Hi, Is there any chance to get this method Async?

     public static int Insert(this Query query, IEnumerable<string> columns, IEnumerable<IEnumerable<object>> valuesCollection, IDbTransaction transaction = null, int? timeout = null);

Thank you very much. This project is awesome!

[ahmad-moussawi]

Would you mind pushing a PR?