Fix issues with batch and command parsing

[shueybubbles]

• If the first line of a batch was blank and the second line had a variable reference in it, we were calculating the offset to the variable incorrectly.
• Identifiers surrounded by [] were not being properly read
• exit command had a buggy regex which matched variants like exit_something

Fixes #197
Fixes #198

[nycdotnet]

Is this going to fix other labels that start with commands such as Rollback_Record: ?

[shueybubbles]

rollback isn't a sqlcmd command so I didn't think it was affected.

[nycdotnet]

@shueybubbles SQLCMD was throwing errors for me on labels like Rollback_Record: and renaming them to something like zRollback_Record: fixed it. Perhaps this is separate - let me try.

[nycdotnet]

OK seems to work for me on this version. 👍

[nycdotnet]

Not a project approver but these changes appear to fix our issues with v0.11

[stuartpa]

Looks good to me (100% code coverage for fix)

[stuartpa]

[nycdotnet]

Thank you for doing this. I know you're dealing with some license generation stuff now and the build is failing, but when you get that squared away, doing a release would be very much appreciated. Thanks for all of your hard work on robustifying go-sqlcmd to work with our legal but gross T-SQL :-)