MLT Query: versatile 'like' parameter

The MLT query has a lot of parameters. For example, a set of documents is
specified with either `like_text`, `ids` or `docs`, with at least one
parameter required. This commit groups all the document specification
parameters under one called `like`. The syntax is described below and could
easily be extended to allow for new means of specifying document input. The
`like_text`, `ids` and `docs` parameters are deprecated.

As a single piece text:

{
  "query": {
    "more_like_this": {
      "like": "some text here"
    }
  }
}

As a single item:

{
  "query": {
    "more_like_this": {
      "like": {
        "_index": "imdb",
        "_type": "movies",
        "_id": "88247"
      }
    }
  }
}

Or as a mixture of all:

{
  "query": {
    "more_like_this": {
      "like": [
        "Some random text ...",
        {
          "_index": "imdb",
          "_type": "movies",
          "_id": "88247"
        },
        {
          "_index": "imdb",
          "_type": "movies",
          "doc": {
            "title": "Document with an artificial title!"
          }
        }
      ]
    }
  }
}

Closes #8039