Add another previewRefreshed trigger that is not debounced

I need this trigger for the mathjax plugin. The `previewRefreshed` trigger already exists, but this is called in a debounced function which causes the mathjax plugin to flicker. I've made a recording of my screen to show the the flickering in the preview, and how it good looks with this pull request:
http://www.screenr.com/TxiH