title: migrations: t.references doesn't allow index name to be specified
body: <p>I have the following in a migration:</p>
  
  <pre><code>create_table :model_with_a_long_name do |t|
    t.references :other_model_with_an_equally_long_name, index: true
  end
  </code></pre>
  
  <p>That produces an index with too long of a name for Postgres.</p>
  
  <p>Is there a way to manually specify the index name (without adding the integer column and the index separately)?</p>
  
  <p>Something like the following:</p>
  
  <pre><code>create_table :model_with_a_long_name do |t|
    t.references :other_model_with_an_equally_long_name, index: true, index_name: 'model_and_other'
  end
  </code></pre>
  
  <p>?</p>