Extend at2x to add proper background-size

> I finally got around and ported [my PR](https://github.com/reworkcss/rework/pull/151) from old core rework to the new plugin structure.

Using background-size: contain doesn't work for many cases in which the size has been specified or the container has a different size.

I've extended the at2x plugin to check the actual image size, halve it and use it as background-size.