Possibility to tune disqusIdentifier

When moving my blog from Jekyll to DocPad I faced the need to preserve Disqus comments which I already had to my posts. But I used different code to embed Disqus which did not provide `window.disqus_identifier`. Thus, to preserve comments I need now to define `window.disqus_identifier` as `undefined` for my older posts. I decided that this would be not very nice to modify `@document.slug` for this. So, I think this would be much better to have optional `disqusIdentifier` attribute which takes care of it.