title: insert to specific index for mongo array
body: <p>Mongo supports arrays of documents inside documents. For example, something like</p>
  
  <pre><code>{_id: 10, "coll": [1, 2, 3] }
  </code></pre>
  
  <p>Now, imagine I wanted to insert an arbitrary value at an arbitrary index</p>
  
  <pre><code>{_id: 10, "coll": [1, {name: 'new val'}, 2, 3] }
  </code></pre>
  
  <p>I know you can update values in place with $ and $set, but nothing for insertion. it kind of sucks to have to replace the entire array just for inserting at a specific index.</p>