Support for UI customizations

[bhch]

Features:

• [ ] Setting custom html class names on form elements
• [ ] Providing custom text for button labels

---

Possible Solution:

Creating a new keyword called ui for this purpose:

Example:

{
    "type": "object",
    "properties": {
        // ...
    },
    "ui": {
        "htmlClass": "custom-class",
        "addButtonText": "Add key",
        "removeButtonText": "Remove key",
        // etc ...
    }
}