Added configuration for customizing default header template

In this release, topper introduces a new configuration parameter for customizing the default header template. The default header template is used when a custom header template is not added for a particular language. The earlier versions of topper used a hard-coded default header template, this has been externalized for the user to customize.

One thing to consider is that the language ID used for the default header template has to be default and it should not be tinkered with!!