Add the `field_value_factor` function to the function_score query

The `field_value_factor` function uses the value of a field in the
document to influence the score.

A query that looks like:
{
  "query": {
    "function_score": {
      "query": {"match": { "body": "foo" }},
      "functions": [
        {
          "field_value_factor": {
            "field": "popularity",
            "factor": 1.1,
            "modifier": "square"
          }
        }
      ],
      "score_mode": "max",
      "boost_mode": "sum"
    }
  }
}

Would have the score modified by:

_score * square(1.1 * doc['popularity'])