title: available CRAN vignettes
body: <p>There's the <code>available.packages()</code> function to list all packages available on CRAN.  Is there a similar function to find all available vignettes?  If not how would I get a list of all vignettes and the packages they're associated with?</p>
  
  <p>As a corner case to keep in mind the <code>data.table</code> package has 3 vignettes associated with it. </p>
  
  <p><strong>EDIT:  Per Andrie's response I realize I wasn't clear.  I know about the vignette function for finding all the available local vignettes, I'm after a way to get all the vignettes of all packages on CRAN.</strong></p>