more flexible 0% step keyframes fix

Thank you so much for making this port. Loving it!

I was doing some testing and found that @keyframes with grouped selector steps like '0%,100%' were coming out as '0,100%'.

So, this pull request will happily deal with that common scenario and it even works for odd edge cases like...

@keyframes name{0,to,from{...}to{...}};
@keyframes name{100%,to,from{...}to{...}};
@keyframes name{100%,0,to{...}to{...}};
@keyframes name{from,0,to{...}to{...}};
@keyframes name{from,to,0{...}to{...}};
@keyframes name{from,to{...}to{...}};

Please let me know if you see anything you want to tweaked or if there are any tests I can help with. Very happy to help. :D

PS - I had tried most of the popular PHP minifiers out there and they all had serious problems. I was dreading the prospecting of having to code my own from scratch so you can imagine how happy I was to discover your code! Thank you so much. ^_^