Publish limits via MQTT

[longzheng]

Fixes #62

Sample output

{
    "opModEnergize": {
        "source": "sep2",
        "value": true
    },
    "opModConnect": {
        "source": "sep2",
        "value": true
    },
    "opModExpLimW": {
        "source": "sep2",
        "value": 1500
    },
    "opModImpLimW": {
        "source": "sep2",
        "value": 1500
    }
}