title: Replace CodeIgniter Query builder
body: <p>I'm trying to make my own implementation of the Query builder in codeigniter. 
  Basicly I would like to add some function and all the provided function. Something like :</p>
  
  <pre><code>$this-db-&gt;from('myTable')
          -&gt;where('id', 1)
          -&gt;custom_where('name', 'customsValues')
          -&gt;get()
  </code></pre>
  
  <p>There values are irrelevent here.
  I've already built a class that extends en current <code>CI_DB_query_builder</code> but I have no idea where to set my class to be used as the primary query builder. I've try to look up similar problem on google but could not find anything.</p>
  
  <p>Any help would be appreciated.</p>