This adds support for search time mappings to the _search API like so:
```
curl -XPOST -HContent-Type:application/json -uelastic:password 'localhost:9200/test/_search?pretty&error_trace' -d'{
  "mappings": {
    "properties": {
      "cat": {
        "type": "script",
        "runtime_type": "keyword",
        "script": "value(\"cat\")"
      }
    }
  },
  "docvalue_fields": ["cat"]
}'
```

It just lifts the `mappings` field from the create index and mappings
update requests and drops it into the `_search` request. It then gives
each mapped field a chance to convert itself into a runtime field. Most
mapping can not and will throw an exception that is translated into an
HTTP 400 response. The `script` converts itself properly and is
available at runtime.