title: Repeat headers when using xtable with longtable option
body: <p>Is there a way to repeat the top row / set headers when generating an xtable with a longtable option ?</p>
  
  <p>For eg., if I have</p>
  
  <pre><code>tableSb &lt;- xtable(df, caption="A Very Long Table", label="ALongTable")
  print(tableSb, include.rownames=TRUE, tabular.environment="longtable", floating=FALSE)
  </code></pre>
  
  <p>This works fine, but when the tables roll over into a new page the headers are not repeated. Any suggestions ?</p>