Add an option to build image URL with one single {width*pixel_ratio} placeholder instead of separated {width} and {pixel_ratio}

Often, the same image could be served for a width of 200px and a pixel_ratio of 2 or a width of 400px and a pixel_ratio of 1.

Cases when this doesn't apply:
- art direction where the 200px / x2 image focuses on one part of the 400px / x1 one
- different compression ratios, more aggressive for the x2 one (cf [compressive image](http://filamentgroup.com/lab/rwd_img_compression/))