feat(helpers): add JSON-LD @graph support

[Kikobeats]

A JSON-LD structure can be defined using @graph property rather than just a single node.

{
  "@context": {
    "generatedAt": {
      "@id": "http://www.w3.org/ns/prov#generatedAtTime",
      "@type": "http://www.w3.org/2001/XMLSchema#dateTime"
    },
    "Person": "http://xmlns.com/foaf/0.1/Person",
    "name": "http://xmlns.com/foaf/0.1/name",
    "knows": {"@id": "http://xmlns.com/foaf/0.1/knows", "@type": "@id"}
  },
  "@id": "http://example.org/foaf-graph",
  "generatedAt": "2012-04-09T00:00:00",
  "@graph": [
    {
      "@id": "http://manu.sporny.org/about#manu",
      "@type": "Person",
      "name": "Manu Sporny",
      "knows": "https://greggkellogg.net/foaf#me"
    }, {
      "@id": "https://greggkellogg.net/foaf#me",
      "@type": "Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/about#manu"
    }
  ]
}

This PR normalizes the @graph in case it's present as a collection of objects, so metascraper can use them for finding a specific property.

closes #233

[coveralls]

Coverage increased (+0.007%) to 97.887% when pulling 4350e07 on next into 2062907 on master.