Add support for MSSQL batch statements (GO) using the existing support in go-mssqldb

[ericsampson]

Currently, any MSSQL migration scripts that contain batches (using the GO statement) cause the script to fail with an error due to invalid syntax.

This is a very common pattern in MSSQL scripts, and so it would be handy if it was supported.

The underlying go-mssqldb driver appears to have support for scripts containing batches, by preprocessing the script to separate it into an array of strings, which can each be sent separately.
https://github.com/denisenkom/go-mssqldb/blob/master/batch/batch.go

The feature request would be to support scripts containing batches, by leveraging this functionality.

I might be able to help with this when I get some free time.
Thanks!

[rabryst]

Also mentioned in #466.