Support declarative custom elements

[kethinov]

The web standard for custom elements doesn't support a declarative way of creating them entirely from within markup. https://www.html5rocks.com/en/tutorials/webcomponents/customelements/

I propose Teddy should support something like:

<template registerElement='x-foo'>
  <p>sample markup structure</p>
  <p>${varname} or {varname}</p>
</template>

If that markup is parsed by Teddy, then it becomes possible to use the custom element <x-foo> like so:

<x-foo varname='hello world'></x-foo>

Or:

<x-foo>
  <varname>hello world</varname>
</x-foo>

Resulting in the following rendered template:

<p>sample markup structure</p>
<p>hello world</p>

This should probably be done separately from teddy as a polyfill for some declarative web component W3C spec, then included in Teddy.