FR: pluggable comments

cryogen supports disquss comments - really nice.
Having a choice of which 'discussion platform' to use, would be nicer: disqus, FB comments, LiveFyre, ... .
A first quick idea is to change the config and remove all 'disqus-*' keys. I guess it would be more something like:
```
:comments yes,
:comments-fn "disqus.js"
```
Well, it would need some more logic, but it wouldn't be too much of a hassle to point people where to change some placeholders in the js-function. Or the `comments-fn` needs to be something different (e.g., pointing to a file and a map of placeholders to replace in that file?). Cryogen could come with a few predefined discussion platforms, but people could add any other one if they like so.