@craigspaeth: Allow markdown conversion to be configured to optionally allow HTML through

So we apparently *do* have a need for allowing HTML input by trusted sources. This swaps out the Markdown parser... again. markdown.js doesn't let you configure it to allow HTML through.  [marked](https://github.com/chjj/marked) does (+ it's alot faster, which is nice)

So this swaps em out and lets us configure the field now: `@model.mdToHtml('foo', sanitize: false)`