Adds support for html fragments much like markdown

This extends Punch to support a html fragment content parser that behaves much like the markdown parser but it simply passes the html fragments through. So instead of `contents/_index/_intro.markdown` you can use `contents/_index/_intro.html` .

I intend to use this to convert an existing static http://foundation.zurb.com/ site so that I can reuse header/footer navigation html around existing page html fragments and I didn't want any inadvertent markdown interpretation.

Considering today is the first day I've used Punch, please review these changes with a generous degree of n00b tolerance as I admit I don't fully understand why I have made all the changes I have.