title: Sequelize migration add "IF NOT EXISTS" to addIndex and addColumn
body: <p><strong>Is there a way to force Sequelize.js to add <code>IF NOT EXISTS</code> to the Postgres SQL created by the <code>queryInterface.addColumn</code> and <code>queryInterface.addIndex</code> methods?</strong> </p>
  
  <p>According to the Postgres Docs this is supported for <a href="https://www.postgresql.org/docs/9.6/static/sql-altertable.html" rel="noreferrer">Alter Table Add Column</a> as well as <a href="https://www.postgresql.org/docs/9.6/static/sql-createindex.html" rel="noreferrer">Create Index</a></p>
  
  <p>I have looked through the Sequelize.js docs without any luck, and I have tried to go through the code to figure out how the SQL is generated, but I have not had any luck yet.</p>
  
  <hr>
  
  <h3>A bit of background, or "Why"</h3>
  
  <p>I am trying to create a migration strategy for an existing postgres instance, and I have currently created a Sequelize migration set which migrates from "nothing" to the current schema. Now I would like to simply get this up and running on my production server where all of the data already exists such that the next time I create a migration, I can run it.</p>
  
  <p>All of this works well for every <code>queryInterface.createTable</code> because the <code>IF NOT EXISTS</code> is automatically added.</p>