Update DOM reference when `opt.el` is added to DOM.

When `opt.el` is being added to the `this.opts`, it's better to update the reference of `opt.el` to the element being added to DOM tree, so that you can modify the list item element later.

-----
i.e when user searches, you may want to wrap the substring with span to highlight.


![](http://f.cl.ly/items/010R1g3B2d1n0h1U3i15/substring.gif)

Currently to make this work:

```javascript
var term = 'item';
var name = 'Item name';
var opt = select.get(name);
var pattern = new RegExp('(' + term + ')', 'i');
var substring = '<span class="substring">$1</span>';

for ( var i = 0; i < select.opts.children.length; i += 1 ) {
  var existingEl = select.opts.children[i];
  if (opt.el == existingEl) {
    existingEl.innerHTML = opt.name.replace(pattern, substring);
    opt.el = existingEl;
  }
}
```

But with this fix, you can do:

```javascript
var term = 'item';
var name = 'Item name';
var opt = select.get(name);
var pattern = new RegExp('(' + term + ')', 'i');
var substring = '<span class="substring">$1</span>';

opt.el.innerHTML = opt.name.replace(pattern, substring);
```