When using show() it should use the original opacity value and default it to 1.

I was having a problem when using `show()` where Zepto always forced the opacity of the element that I'm animating to 1. But there are cases when I already have an opacity value pre defined and want to keep that. I can do that with jQuery but with Zepto I would have to do something like `$('.class').show().css('opacity', .5);`
This is a really small implementation but I think it's a good addition to the `fx` module.