title: Need io.ReaderAt from zip archive entry (the entry is a nested .xlsx file)
body: <p>Let me preface this question with the fact that I already know that an excel 2007 file is itself a .zip file, renamed to .xlsx.</p>
  
  <p>Ok, now that you know that here's the deal.  I'm trying to extract an Excel 2007 file from within a .zip archive all in memory. I can't (rather, I <strong>really</strong> don't want to) extract the whole archive to disk and then work with the .xlsx file from there. </p>
  
  <p>The problem is that our method of reading excel 2007 files requires a ReadAt method (such as what is defined by <a href="http://golang.org/pkg/io/#ReaderAt" rel="noreferrer">io.ReaderAt</a>). Unfortunately, the archive/zip package exposes an interface for <a href="http://golang.org/pkg/archive/zip/#File.Open" rel="noreferrer">zip file entries</a> that only gives back <a href="http://golang.org/pkg/io/#ReadCloser" rel="noreferrer">io.ReadCloser</a>. </p>
  
  <p>Is there any way to get around this situation? Again, I'd like to do this all in memory, without flushing to disk at all.</p>