[Request] Replace child node

Currently we are able to remove specific child and add new child in the tail of the parent, but we can't replace specific tag node.

sunra/php-simple-html-dom-parser can do this.

``` php
$child->outertext = '<new />';
```

Here is my thought:

``` php
$child = $parent->find('child')[0];
$newChild = new Tag('new');
$parent->replaceChild($child->id(), $newChild);
```

Thanks!
