title: is it possible to query workitem custom field in TFS SDK
body: <p>I have few workitems which contain custom field called "Reference ID"
  is it possible to query using wiql on this custom field.
  Currently I am using the following approach:</p>
  
  <p>//foreach project in TFS</p>
  
  <p>//form the wiql</p>
  
  <pre><code>WorkItemCollection workItemCollection = workItemStore.Query(
                      " SELECT [System.Id], [System.WorkItemType]," +
                      " [System.State], [System.AssignedTo], [System.Title] " +
                      " FROM WorkItems " +
                      " WHERE [System.TeamProject] = '" + tfsProject.Name +
                      "' ORDER BY [System.WorkItemType], [System.Id]");
  </code></pre>
  
  <p>//run a loop against the result set</p>
  
  <p>//if workitem.Fields["Reference ID"]=required value </p>
  
  <p>//do some tasks on this workitem</p>
  
  <p>this approach is taking quite sometime since there are more than 1000 results.</p>
  
  <p>my question:
  is it possible to add custom field also as a filter condition in the above query</p>