Support for sqlite_backup

I know this is not going to be accessible from sql.Driver interface but it may be useful if we can simply import "modernc.org/sqlite" or "modernc.org/sqlite/lib" and do the backup.

See a reference implementation here: https://github.com/mattn/go-sqlite3/blob/4d8693d6ee/backup.go

Do you think this may be considered a must-have feature?