title: Seek in buffered reader
body: <p>I need to create a buffered reader of an existing child <code>io.Reader</code>, but that reader must support seeking in data already read and buffered from the child.</p>
  
  <p>So when <code>n</code> bytes were already read, I want to be able to reset the reader to offset <code>0</code> and read that chunk again.</p>
  
  <p>Unfortunately <code>bufio.Reader</code> doesn't support seeking.</p>
  
  <p>Is there a standard reader that supports this, or do I have to implement my own?</p>