title: substream from istream
body: <p>Suppose I have an <code>ifstream</code> which represents a large file containing lots of sub-files aggregated together. I want to be able to create a "sub" <code>istream</code> from the larger <code>ifstream</code> (given a size and offest) representing a part of the file so other code can read from that substream as if it was an independent <code>istream</code>.</p>
  
  <p>Any ideas on how I might accomplish this?</p>
  
  <p><strong>EDIT</strong>
   - I would prefer to avoid boost.</p>