Support AngularJS templates

When you develop an AngularJS app you need to set type="text/ng-template" customizing the html bundle:

```C#
bundles.Add<HtmlTemplateBundle>("views/templates", 
                          c => { c.ContentType = "text/ng-template"; });
```

This only works in Debug. In Release you receive an exception (KeyNotFoundException) because that type isn't in the FileExtensionsByContentType Dictionary.