Add versatile 'like' parameter

[alexksikes]

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!"
          }
        }
      ]
    }
  }
}

[s1monw]

I like this thsi looks much better! Can we maybe have some comments next to the parse fields which of them are deprecated?

[s1monw]

LGTM - I wonder if we should wait with this a bit and refactor ParseField to support field names where all of them are deprecated. I'd love to be able to pass something like this:

ParseField LIKE_TEXT = new ParseField("like_text", true /* deprecated */, "like" /* the actual field to use instead */);

such that we can barf with the strict parse option? (note my example might not be the best way to implement it)

[alexksikes]

OK I've just opened #8067