Conflict/Duplicate handling

Noticed that activerecord-import was used, and was wondering if there are any plans to support their on duplicate options especially for databases like postgres and MySQL. I would be interested in submitting a PR for this, something that matches the current [on duplicate key options that activerecord-import already supports](https://github.com/zdennis/activerecord-import#duplicate-key-ignore).

The issue links activerecord-import's duplicate-key-ignore options and specifically names PostgreSQL and MySQL. No project files, tests, or entry points are identified, so start by reviewing the linked options and determining how the plugin currently imports records. Done means an agreed, tested duplicate-handling interface for the requested databases.

索引モデルが issue の本文から書いたものです。