Expose the table name being migrated

[HeroesDieYoung]

The table name being operated on is not exposed at runtime through the dyno object. If developers are using TypeScript, the table name is a required part of the update expression. So, to avoid the compilation error, developers must provide their migrations another way to get the table name being worked on - duplicating it with the command line arguments used to invoke dynamodb-migrator.

If the dyno object made the table name string available, we'd be able to avoid this extra step.