Allow to customize label and value array keys

[rodrigopedra]

I have a similar structure but much over complicated on a project.

Unfortunately the frontend expects the serialized data to be keyed like

[
  {"id": 1, "name": "Mary"},
  {"id": 2, "name": "John"}
]

Instead of using the keys label and value provided by this package.

This PR:

• Adds a configuration file to allow customizing the serialization keys for the label and value properties
• Add a relevant test case

[rubenvanassche]

Looking great thanks!