[Feature] High volume threshold

I thought it would be useful to have the volume color change when going over 100%, or any value set with high_threshold for that matter. I am not familiar with the code, so making a pr with accaptable changes would take me a long time, but I made a demonstration how it would look like: https://github.com/g-rden/i3status/commit/829258863ab20f94e6c548955dca13f63aedbc89

Is this something that could be added? And if so, does anyone have a way to quickly implement it, or should I look into it?