Can CustomElement+Builtin be added (using ungap's modules) ?

It would be nice to add [CustomElement](https://github.com/WebReflection/custom-elements) and [CustomElementBuiltin](https://github.com/WebReflection/custom-elements-builtin) polyfills.
They work very well. Note that they don't polyfill shadow-dom.

[Example of detection](https://github.com/pageboard/server/blob/master/packages/polyfill/index.js#L17-L36)