title: Validate ExtJS CheckboxGroup checked boxes
body: <p>Using ExtJS 4, I have a <code>Ext.form.CheckboxGroup</code> and I want to do validation on this checkbox group like 'valid only when 1, 2, or 3 checkboxes checked' out of a set of maybe 10 checkboxes.  There is an existing <code>allowBlank: false</code> config option I can set to ensure the CheckboxGroup has at least one checkbox checked.  I'd like to get essentially the same functionality but with a custom validator function so that I can specify my own criteria.</p>
  
  <p>How can I accomplish this?</p>
  
  <p>Looking at the docs and code, I don't see a clear way to do this.  Any suggestions?</p>