Option "Extend" has been added.

Useful when you are going to extend some parameters of object, but some fields keep without changing. For example manifest:
{
  "app": {
    "override": "override",
    "keep": "keep"
  }
}
and you want to override field "override" but keep field "keep" without changes.
Now with only option "Override" you only can override whole section "app".
But with new option "Extend" you can extend this object with keeping some fields.

Tests also has been added.