support query mode

[mcwhittemore]

Right now we support scan and stream. While we have filter functionality to let you transform a stream so the migration will only run on some items in the export, we can only use this where a database export is usable.

To support filtered results where an export is not viable, we should consider supporting the query operation.

This would require the migration file to export a query attribute. I'm torn on if it should be a function that accepts a dyno instance or a JSON object that describes the dynamo query.