add intersectIntervals function

[seansfkelley]

I think this would work:

function intersectIntervals(a: Interval, b: Interval, defaultInterval: Interval): Interval {
    const [ left, right ] = a.min <= b.min ? [ a, b ] : [ b, a ];
    if (left.max < right.min) {
        return defaultInterval;
    } else {
        return {
            min: Math.max(left.min, right.min),
            max: Math.min(left.max, right.max)
        };
    }
}