Add comments to entity

[luisherranz]

Comments appear in the replies array of the _embed property:

We should return an array of comments:

connection.entity('post', 60).comments // => array of comments

We need a new Comment model. Comments in the REST API have the following schema:

{
  "id": 49719,
  "post": 117741,
  "parent": 49714,
  "author": 0,
  "author_name": "M.A.",
  "author_url": "",
  "date": "2018-08-13T23:09:00",
  "date_gmt": "2018-08-13T21:09:00",
  "content": {
    "rendered": "<p>Toda la razón.<br />\nPero si el tiempo ha demostrado algo es que por mucho que cambies, unos están arriba y otros abajo. Y casi siempre son los mismos&#8230;.</p>\n"
  },
  "link": "https://www.diariomotor.com/competicion/noticia/zak-brown-no-es-ideal-tener-a-dos-pilotos-jovenes-en-mclaren/#comment-49719",
  "status": "approved",
  "type": "comment",
  "author_avatar_urls": {
    "24": "https://secure.gravatar.com/avatar/09371c7b05ce82b1f692a19cd164a60e?s=24&d=mm&r=g",
    "48": "https://secure.gravatar.com/avatar/09371c7b05ce82b1f692a19cd164a60e?s=48&d=mm&r=g",
    "96": "https://secure.gravatar.com/avatar/09371c7b05ce82b1f692a19cd164a60e?s=96&d=mm&r=g"
  },
  "meta": [],
  "_links": {
    "self": [
      {
        "href": "https://www.diariomotor.com/competicion/wp-json/wp/v2/comments/49719"
      }
    ],
    "collection": [
      {
        "href": "https://www.diariomotor.com/competicion/wp-json/wp/v2/comments"
      }
    ],
    "up": [
      {
        "embeddable": true,
        "post_type": "noticia",
        "href": "https://www.diariomotor.com/competicion/wp-json/wp/v2/noticias/117741"
      }
    ],
    "in-reply-to": [
      {
        "embeddable": true,
        "href": "https://www.diariomotor.com/competicion/wp-json/wp/v2/comments/49714"
      }
    ]
  }
}

We need to decide what fields to expose directly (the other ones can be got from .raw)