Add support for `ref` attribute

[miyconst]

Right now imported-template can load content from a url or import html provided as content property.

It would be really nice to be able to import other templates like this:

<template id="my-template">
    <div>Some HTML here</div>
</template>
<template is="imported-template" ref="my-template"></template>

[miyconst]

As agreed with @tomalec and @warpech it should be another element, not a feature inside imported-template.

[miyconst]

@tomalec this is currently supported, right?

[tomalec]

no, it's not.