title: Specify custom index name when using add_reference
body: <p>I have the following migration</p>
  
  <pre><code>class LinkDoctorsAndSpecializations &lt; ActiveRecord::Migration
    def up
      add_reference :doctors, :doctor_specialization, polymorphic: true, index: true
    end
  
    def down
      remove_reference :doctors, :doctor_specialization, polymorphic: true
    end
  end
  </code></pre>
  
  <p>when i run <code>rake db:migrate</code> i am getting the error</p>
  
  <p><code>Index name 'index_doctors_on_doctor_specialization_type_and_doctor_specialization_id' on table 'doctors' is too long; the limit is 63 characters</code>  </p>
  
  <p>so how can i specify the index name when using add_reference like the way we specify in <code>add_index :table, :column, :name =&gt; 'index name'</code></p>