Missing indexes helper

ping @tjoyal @sgnr 

```ruby
RescueFromDuplicate.missing_indexes
=> [#<RescueFromDuplicate::MissingIndex:0x007fc6f69d2938
  @attributes=[:title],
  @columns=["shop_id", "title"],
  @model=
   ShopPolicy>,
...
```
avoid nesting here https://github.com/Shopify/activerecord-rescue_from_duplicate/blob/master/lib/rescue_from_duplicate/active_record/extension.rb#L3