Datalist polyfill: Aligned polyfill with v4.0 model for plugins

Still needs work.

TODO:
* How to include polyfill SCSS? Currently not included. Should it be AJAXed in as separate file, injected as style element in head (so bundled into JS), or other. Would prefer to avoid the v3.1 approach where CSS got bundled into the core CSS regardless of whether it was needed or not.
* How to do examples for polyfills? Should a .hbs file be used like with plugins?