Proposal: accepting a message handler

[tc-hib]

Hello,

Sometimes it would be useful to receive messages from the server.

• Using BACKUP DATABASE with STATS option, we can track its progress
• When a stored proc does many things for a long time, we may want it to say what it's doing

If we want to show this information in some UI, we need a way to handle it, we can't have it in logs only.

I propose that we add a new specific type of query argument, like ReturnStatus. That type would be a callback function that would take a mssql.Error.

Here is an example of how it could be implemented: https://github.com/denisenkom/go-mssqldb/compare/master...tc-hib:pr-message-handler?expand=1

[tc-hib]

I forgot there was #221 already.

@kardianos is there any reason why you didn't add support for sqlexp.ReturnMessage in go-mssqldb?