Add @id property to make Schema.org extensible

Good that Gost implements a basic level of schema.org structured data.

However, the entities generated in the schema are missing an @id attribute. This makes them non extensible.

When a JSON-LD object has an @id attribute one can always add attributes to the object by creating a second object with the same @id. That way objects generated by the Ghost core could be extended via header injections, or HTML blocks.

For example Ghost generates an Organization as publisher of articles. If the Organization would have an @id = https://example.com/#organization attribute, I could add an address and other attributes with

<script type="application/ld+json">
{
  "@context": "https://schema.org",
  "@type": "Organization",
  "@id": "https://example.com/#organization",
  "address": {
    "@type": "PostalAddress",
    "addressLocality": "Paris, France",
    "postalCode": "F-75002",
    "streetAddress": "38 avenue de l'Opéra"
  },
  "email": "secretariat(at)google.org",
  "faxNumber": "+33142685301"
}
</script>