Added MIME type for open web app manifests

As a Phusion Passenger user who deploys to Heroku, I have struggled to serve certain static assets because editing the Nginx configuration file on Heroku during slug compilation to support the additional MIME types is a huge pain. Adding the MIME types to Passenger itself seems like a better solution overall than resorting to elaborate erb-based templates that may or may not work from version to version.

This pull request adds support for [open web app manifest](https://developer.mozilla.org/en-US/Apps/Build/Manifest) files.