Support refresh rate

[koiuo]

Rate should be handled by randrctl.

1. matching by rate is not necessary. It is so rare, that edid should be enough
2. in outputs section it should be optional. If absent, don't pass --rate to xrandr and allow it to chose the best rate automatically
3. do not dump rate by default. Do that only if specific option is passed to dump command

    "outputs": {
        "LVDS1": {
            "pos": "0x312",
            "panning": "0x0",
            "mode": "1366x768",
            "rotate": "normal"
        },
        "DP1": {
            "pos": "1366x0",
            "panning": "0x0",
            "mode": "1920x1080",
            "rotate": "normal",
            "rate": 70
        }
    }