Add tags for added and removed content in jsondiff

[jsoriano]

It is not clear what is added and removed in jsondiff introduced in #743.

Add +/- tags before content added or removed as indication of the reason
to appear in the diff.

Changes appear now like this:

Added:

{
    "expected": [
        {
            ...skipped 5 object properties...,
            "source": {
                + "address": "127.0.0.1",
                ...skipped 1 object property...
            },
            ...skipped 4 object properties...
        },
        ...skipped 8 array elements...
    ]
}

Removed:

{
    "expected": [
        {
            ...skipped 5 object properties...,
            "source": {
                ...skipped 2 object properties...,
                - "port": "33456"
            },
            ...skipped 4 object properties...
        },
        ...skipped 8 array elements...
    ]
}

Changed:

{
    "expected": [
        {
            ...skipped 5 object properties...,
            "source": {
                "address": "127.0.0.2" => "127.0.0.1",
                ...skipped 1 object property...
            },
            ...skipped 4 object properties...
        },
        ...skipped 8 array elements...
    ]
}

An object is renamed:

{
    "expected": [
        {
            ...skipped 5 object properties...,
            + "source": {
                + "address": "127.0.0.1",
                + "ip": "127.0.0.1"
            + },
            - "sources": {
                - "address": "127.0.0.1",
                - "ip": "127.0.0.1"
            - },
            ...skipped 4 object properties...
        },
        ...skipped 8 array elements...
    ]
}

Reported in #743 (comment).

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