title: add list section headers with PagedListAdapter / PagedList
body: <p>I'd like to headers in a list using the <code>android.arch.paging</code> components.</p>
  
  <p><a href="https://i.stack.imgur.com/zNdWE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zNdWE.png" alt="enter image description here"></a></p>
  
  <p>Usually this would easy enough, just add different types for a <code>RecyclerView</code> adapter to handle and create a new list from items including headers.</p>
  
  <p>But with the paging components I'm essentially handing the <code>PagedList</code> a <code>PositionalDataSource</code> from an <code>SQL</code> query.  Is it possible to interrupt this and add header types?</p>