Adding support for Hogan (.hgn)

Just changed the preferences to support [Hogan](http://twitter.github.io/hogan.js) files (`.hgn`).
[Hogan](http://twitter.github.io/hogan.js) is a templating engine created by Twitter and follows the same [syntax/specification](http://mustache.github.io/mustache.5.html) of [Mustache](http://mustache.github.io).