title: dataTables - filter/search only in selected rows
body: <p>I have a question. Is there any possibility in jQuery dataTables plugin to only filtering rows with specific class?
  For example: I got a table and I want to filter only rows with <code>searchable</code> class. The rest rows stays as they is.
  Is this even possible?</p>
  
  <pre><code>&lt;table class="dataTable"&gt;
    &lt;tr class="searchable"&gt;
      &lt;td&gt;text to search&lt;/td&gt;
      &lt;td&gt;text to search&lt;/td&gt;
    &lt;/tr&gt;
    &lt;tr&gt;
      &lt;td&gt;something else&lt;/td&gt;
      &lt;td&gt;something else&lt;/td&gt;
    &lt;/tr&gt;
    &lt;tr class="searchable"&gt;
      &lt;td&gt;text to search&lt;/td&gt;
      &lt;td&gt;text to search&lt;/td&gt;
    &lt;/tr&gt;
    &lt;tr&gt;
      &lt;td&gt;something else&lt;/td&gt;
      &lt;td&gt;something else&lt;/td&gt;
    &lt;/tr&gt;
  &lt;/table&gt;
  </code></pre>
  
  <p>Edit:</p>
  
  <p>I'm thinking to write my own plugin, but I never do that before.
  Something like:</p>
  
  <pre><code>getFilterInput
  table.each(tr).function(){
  if(row.hasClass(searchable){
      //do filtering
  }else{
      //leave row alone
  }
  </code></pre>
  
  <p>Anyone does something like that and can give me a clue where to start?</p>