Support Handlebars templates

When you develop an Ember.js app you need to set `type="text/x-handlebars"` customizing the html bundle:

    bundles.Add<HtmlTemplateBundle>("views/templates", c => { c.ContentType = "text/x-handlebars"; });

This only works in Debug. In Release you receive an exception (KeyNotFoundException) because that type isn't in the FileExtensionsByContentType Dictionary.

I have also added the common "text/x-handlebars-template" type that is shown on the Handlebars website examples.