Template namespace

Pull request for the opened issue : https://github.com/tonai/grunt-build-html/issues/3.
Add the templateNamespaceRoot options.

Without it, it will simply load all templates with their filename as template key (regardless of the templates folder structure).
If you set the templates root folder, it will serve as the key origin :
template : test/fragments/subfolder/my-content.html
templateNamespaceRoot : test/fragments
template key : subfolder/my-content