Concatenated partials/templates with attached scopes

To boost performance imported-template supports concatenated partials, and stamps all "root" templates all together.

To mark isolated data scope for each partial, you may use <imported-template-scope> custom element.

<imported-template
    href="./partials/concatenated_scoped.html"
    model='{
        "vendorA": {"data": "from scope of Vendor A"},
        "vendorB": {"data": "from scope of Vendor B"}
    }'
    ></imported-template>