Better way to delegate the build of delite/handlebars to require-js/text.

I don't know why I didn't think about this at the time but `pluginBuilder` is a cleaner way to delegate the build of `delite/handlebars` to `requirejs-text/text`.

This allow to simplify the load function as it will not be called by the build. The build will call the load function of requirejs text instead.