title: Migration for changing on_delete option on references column
body: <p>I was playing with Phoenix and making has_many associations. I usually do <code>on_delete: :delete_all</code> as an option to the reference column. But if I change my mind and want to change it later for <code>nilify_all</code>, is there a way to this with inside a migration? </p>
  
  <p>Migration for creating the table: </p>
  
  <pre><code>  def change do
      create table(:messages) do
        add :body, :text
        add :sender_id, references(:users, on_delete: :delete_all)
  
        timestamps()
      end
      create index(:messages, [:sender_id])
  
    end
  </code></pre>
  
  <p>I'm looking for something like this:</p>
  
  <pre><code>def change do
    change_options table(:messages), :user_id, on_delete: :nilify_all
  end
  </code></pre>
  
  <p>I've seen <a href="https://hexdocs.pm/ecto/Ecto.Migration.html#modify/3" rel="nofollow noreferrer">modify</a> and <a href="https://hexdocs.pm/ecto/Ecto.Migration.html#alter/2" rel="nofollow noreferrer">alter</a> but I didn't see anything about on_delete. Maybe it's not possible via migrations? </p>