feat: add temperatureOffset, humidityOffset and percentage-based offset config options

[Aldarande]

Problem

DHT sensors placed inside a MagicMirror enclosure are often affected
by the heat generated by the display and electronics, resulting in
inaccurate temperature and humidity readings.

Solution

Add four optional config parameters to compensate for sensor drift:

• temperatureOffset (default: 0): fixed value added to the raw temperature reading
• temperatureOffsetPercent (default: 0): percentage applied to the raw temperature reading
• humidityOffset (default: 0): fixed value added to the raw humidity reading
• humidityOffsetPercent (default: 0): percentage applied to the raw humidity reading

Both fixed and percentage offsets can be combined:
final value = raw * (1 + offsetPercent / 100) + offset

Usage

{
    module: "MMM-DHT-Sensor",
    config: {
        sensorPin: 2,
        sensorType: 22,
        temperatureOffset: -3,       // fixed offset: -3°C
        temperatureOffsetPercent: 0, // percentage offset
        humidityOffset: 0,
        humidityOffsetPercent: -5,   // sensor reads 5% too high
    }
}

Changes

• MMM-DHT-Sensor.js: added four offset options to defaults and applied
  them in processSensorData()