Adding option for color threshold

In my project "canopsis" we needed to change the gauge color depending the given value  when "levelColorsGradient" is false. So I added a new option "levelThresholds". Each time one value is reached, the gauge change to the next color. Then we can repoduce a "warning/criting" gauge threshold.

```javascript
new JustGage({
        id: "ttfb", 
        value: gvalue, 
        min: 0,
        max: 100,
        levelColors: [
          "#a9d70b",
          "#1900FF",
          "#ff0000",
          "#000000"
        ],
        levelThresholds: [60,80,95],
        levelColorsGradient : false,
        title: "test threshold"
}); 
```

* from 0 to 60 it's  #a9d70b
* 60-80: #1900FF ,
* 80-95:#ff0000
* 95+:  #000000