Add support of custom fields attribute

[To-om]

Custom fields attribute is a JSON object that contain any number of fields. Each field contains a nested object where key is the field type. Supported types are: string, number, date and boolean.

This structure make ElasticSearch mapping stable (same field name have always the same type).

Each field may also contain the key order which is used to order them in a list.

Here is an example of valid custom fields:

{
  "fieldName": { "number": 42, "order": 2 },
  "otherField": { "boolean": true, "order": 1 },
  "thirdField": { "date": 1497861727 }
}

Elastic4play accepts field with multiple types (e.g. { "otherField": { "boolean": true, "number": 1 } }.