Add support for `<content>`

[tomalec]

So we could in fact nest imported partial into client template:

<template is="imported-template" content="{{ html }}">
  <h2>Partial is already loaded</h2>
  <ul>
    <content></content>
  </ul>
</template>

Which will produce same result as:

<h2>Partial is already loaded</h2>
<ul>
  <template is="imported-template" content="{{ html }}">
  </template>
</ul>

But <h2> and <ul> won't be visible until partial is loaded.