allow passing sql.TxOptions when using Session.Begin

[stevenctl]

Sometimes when using transactions I want to enforce behavior by setting read-only or isolation level. The current Session.Begin doesn't take any options and when it eventually calls BeginTx to get the underlying sql.Tx it just uses nil for the options.

This change is backward compatible and allows passing sql.TxOptions to BeginWithOpts while Begin retains it's original functionality.

Usage:

tx, err := sess.BeginWithOpts(&sql.TxOptions{
    Isolation: sql.LevelRepeatableRead,
})
defer tx.RollbackUnlessCommitted()

[stevenctl]

Seems odd that the build is failing on SQLi stuff.. didn't touch much at all.

[DoubleDi]

hello! Thanks for your PR. We will fix our CI problems and merge you shortly