update _.random(min, max, fixed)

change `_.random(min, max)` to `_.random(min, max, fixed)` .

If the third parameter exist, it will return a number between min and max, keeping no more than `fixed` decimals. example:

    _.random(10) //4
    _.random(5, 10) //7
    _.random(5, 10, 2) //7.13
    _.random(5, 10, 2) //9.3