title: use extra params in store dynamically
body: <p>I have a form. In the form, I am using a <code>comboBox</code> with a <code>store</code>.</p>
  
  <pre><code>{
      xtype: 'combobox',
      id: 'SubContractor',
      name: 'SubContractor',
      fieldLabel: 'Sub Contractors',
      selectOnFocus: true,
      editable: false,
      displayField: 'FirstName',
      store: 'jsonGetSubContractorsList',
      typeAhead: true,
      allowBlank: false,
      typeAheadDelay: 20,
      valueField: 'SubContractID',
      width: 440,
      labelWidth: 229
  }
  </code></pre>
  
  <p>In store, in proxy I have static <code>extraParams</code>, it's working. </p>
  
  <pre><code>proxy: {              
      type: 'ajax',
      url: '/admin/contract/subcontractors/jsonsubcontractorslist',
      extraParams: {
          cid : 34
      },
      reader: {
          type: 'json',
          root: 'data'
      }
  },
  </code></pre>
  
  <p>But I don't get, how to send the contract ID to my store dynamically.</p>