Add support for multi-fields

**WARNING**

This PR can be merged only if https://github.com/elasticsearch/elasticsearch/pull/6867 PR is merged in elasticsearch core code.

This PR allows the following to work:

```
DELETE /test
PUT /test
{
  "settings": {
    "number_of_shards": 1
  }
}
PUT /test/person/_mapping
{
  "person": {
    "properties": {
      "file": {
        "type": "attachment",
        "path": "full",
        "fields": {
          "file": {
            "type": "string",
            "fields": {
              "store": {
                "type": "string",
                "store": true
              }
            }
          },
          "content_type": {
            "type": "string",
            "fields": {
              "store": {
                "type": "string",
                "store": true
              },
              "untouched": {
                "type": "string",
                "index": "not_analyzed",
                "store": true
              }
            }
          }
        }
      }
    }
  }
}

PUT /test/person/1?refresh=true
{
  "file": "IkdvZCBTYXZlIHRoZSBRdWVlbiIgKGFsdGVybmF0aXZlbHkgIkdvZCBTYXZlIHRoZSBLaW5nIg=="
}

GET /test/person/_search
{
  "fields": [
    "file.store",
    "file.content_type.store"
  ],
  "aggs": {
    "store": {
      "terms": {
        "field": "file.content_type.store"
      }
    },
    "untouched": {
      "terms": {
        "field": "file.content_type.untouched"
      }
    }
  }
}
```

It gives:

```js
{
   "took": 3,
   "timed_out": false,
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "hits": {
      "total": 1,
      "max_score": 1,
      "hits": [
         {
            "_index": "test",
            "_type": "person",
            "_id": "1",
            "_score": 1,
            "fields": {
               "file.store": [
                  "\"God Save the Queen\" (alternatively \"God Save the King\"\n"
               ],
               "file.content_type.store": [
                  "text/plain; charset=ISO-8859-1"
               ]
            }
         }
      ]
   },
   "aggregations": {
      "store": {
         "doc_count_error_upper_bound": 0,
         "buckets": [
            {
               "key": "1",
               "doc_count": 1
            },
            {
               "key": "8859",
               "doc_count": 1
            },
            {
               "key": "charset",
               "doc_count": 1
            },
            {
               "key": "iso",
               "doc_count": 1
            },
            {
               "key": "plain",
               "doc_count": 1
            },
            {
               "key": "text",
               "doc_count": 1
            }
         ]
      },
      "untouched": {
         "doc_count_error_upper_bound": 0,
         "buckets": [
            {
               "key": "text/plain; charset=ISO-8859-1",
               "doc_count": 1
            }
         ]
      }
   }
}
```

Note that using shorter definition works as well:

```
DELETE /test
PUT /test
{
  "settings": {
    "number_of_shards": 1
  }
}
PUT /test/person/_mapping
{
  "person": {
    "properties": {
      "file": {
        "type": "attachment"
      }
    }
  }
}
PUT /test/person/1?refresh=true
{
  "file": "IkdvZCBTYXZlIHRoZSBRdWVlbiIgKGFsdGVybmF0aXZlbHkgIkdvZCBTYXZlIHRoZSBLaW5nIg=="
}

GET /test/person/_search
{
  "query": {
    "match": {
      "file": "king"
    }
  }
}
```

gives:

```js
{
   "took": 53,
   "timed_out": false,
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "hits": {
      "total": 1,
      "max_score": 0.095891505,
      "hits": [
         {
            "_index": "test",
            "_type": "person",
            "_id": "1",
            "_score": 0.095891505,
            "_source": {
               "file": "IkdvZCBTYXZlIHRoZSBRdWVlbiIgKGFsdGVybmF0aXZlbHkgIkdvZCBTYXZlIHRoZSBLaW5nIg=="
            }
         }
      ]
   }
}
```