I am sure that someone asked about this, but now for some unknown reason I can not find this question. Maybe one still need it?
---
Style: white
...
<script>$(function(){
$("pagenum").each(function(i){
var correctedIndex = (i + 1);
$(this).append(
"<center>⊰ Page " +
correctedIndex +
" ⊱</center>")
});
});</script>
<pagenum>
{{my first page.md}}
</pagenum>
<pagenum>
</pagenum>
<pagenum>
{{my third page.txt}}
</pagenum>
==========
Result