Update for modern browsers

- adopt babel-preset-github (drop IE 11)
- stop transforming custom element classes

Author: mislav

README.md

@@ -63,7 +63,6 @@ Browsers without native [custom element support][support] require a [polyfill][]
 - Chrome
 - Firefox
 - Safari
-- Internet Explorer 11
 - Microsoft Edge
 
 [support]: https://caniuse.com/#feat=custom-elementsv1