Possibility to tune disqusIdentifier

[varya]

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.

[mikeumus]

Affects #14 (and #19 closed-merged).

[balupton]

I have become too preoccupied by other things to review, merge, and release this PR.

However, you should have received an invite a while ago to join the DocPad Extras Team, which will give you write access to this repository, so you can merge in the PR.

For an orientation, or if you need any assistance following the semi-automatic release process from CONTRIBUTING.md, reach out via https://bevry.me/discord/dev