added includes method to enable explicit join with associated tables

added `includes(*join_tables)` to include the associated tables explicitly because `references` doesn't know whether the associated tables are being loaded.

otherwise, a ERROR will be thrown for undefined tables. For example
```
class Person
    has_many :phone_numbers

   rails_admin do
     configure :phone_numbers, :has_many_association
     list do
      field :phone_numbers do
        searchable ['phone_numbers.number']
      end
     end
   end
end
```
will throw an error
```
`SELECT  "people".* FROM "people"  WHERE ((LOWER(phone_numbers.location) ILIKE '%13%'))  ORDER BY people.id desc LIMIT 20 OFFSET 0`
```