refactor to work with `express-view`

* the plugin is now a class definition that can be extended easily.
* leverage `Y.Template.register()` and `Y.Template.get()`, available in yui@3.12.x.
* a compiled version of the template is provisioned under the bundle object for runtime on the server side.
* yui output format is now opt-in.
* example does not use `express-yui` anymore, instead it uses `express-view`.
* the `handlebars` component can be controlled from outside by providing a custom version of it when creating an instance of the plugin.