Investigate webcomponents

Another vaguer issue from the backlog but I like to keep it even if it will be closed as soon as we get to it.

• Investigate how ShadowDOM, webcomponents etc. could be used to make MathJax a seamless polyfill.
  • Hide output in ShadowDOM
  • leave (author configured?) source in the LightDOM
  • use mutation observers to detect changes to LightDOM.