Keep context while exporting json-properties

[dhorelik]

Currently terms' context is being lost while exporting key-value json. If we export manually via https://poeditor.com, context is preserved as another nesting level:

{
    "term1": "phrase1",
    "context": {
        "term2": "phrase2",
        "term3": "phrase3"
    }
}

This gets trimmed to

{
    "term1": "phrase1",
    "term2": "phrase2",
    "term3": "phrase3"
}

The PR fixes that and adds context support.