Support operator class on PostgreSQL index creation

The use case for this is for example when you try to create a gin/gist index on a text column. Before this PR I had to do:
```ruby
class AddFulltextIndexesToUsers < ActiveRecord::Migration
  def up
    execute <<-SQL
      CREATE  INDEX "index_users_on_bio" ON "users" USING gin ("bio" gin_trgm_ops)
    SQL
  end

  def down
    execute <<-SQL
      DROP INDEX "index_users_on_bio"
    SQL
  end
end
```

now I can do

```ruby
class AddFulltextIndexesToUsers < ActiveRecord::Migration
  def change
    add_index  :users, :bio,  using: 'gin', opclass: "gin_trgm_ops"
  end
end
```