Add support for `LitElement` (Polymer Skeleton v2-beta)

[LasaleFamine]

After the latest blog post we should take care of this.

Things to do:

1. add template-string-loader
2. switch from PolymerElement to LitElement

[sipi]

Hi,
I have created a fork using LitElement. I'm sending this post to keep you inform about.

Thanks for your works,
regards,
6π

Edit: I'm currently working on a TypeScript version :)